Why do states get called for a certain candidate before 100% in?

It's generally based on exit interviews (and those are notoriously specious).

because when the results are clearly lopsided......it's never happened that the last bit of uncounted votes  ALLLLLLLLL went to the losing candidate,  making the difference. 

the last few uncounted votes, will simply reflect the already established patterns..........60 vs. 40%    70  vs. 30%  and so forth. 

so it's possible to call  "projected"  winners..........but of course,  the final tally will be made. 

The media has been doing that  for decades . 

Calling is something that the media does.  It has no official meaning whatsoever. 

From the legal side, there is only a winner in local races when the local election authority finishes their canvass and certifies the results in that jurisdiction.  In races that cross multiple jurisdictions, there is only a winner when the state election authority has received the results from all of the jurisdictions in the state and completed its canvass of the results.  In short, there is not an official winner of an election until several weeks after the election.

The media, however, is covering more things than the election.  Additionally, on election night, they typically have multiple races that they are covering.  Calling a race is the media's way of saying that we have enough information to know who has won this race so we will not be looking at this race for the rest of the evening -- unless something changes that causes them to retract their call --  and will move to focus on other races that are still competitive.  

The media is able to do that because demographics do matter.  While there are differences of opinions within every significant demographic (women, minorities, religious groups, etc., urban vs. rural), those differences are somewhat predictable.  More importantly, we have tended to segregate ourselves to a large degree.  Thus, we have a historical record of how certain precincts and counties tend to vote.  That means that there are certain precincts and jurisdictions that should be good for candidate X and others that should be good for candidate Y.  If we have 20% of the precincts in and candidate X is winning strongly in the precincts that she should win and candidate Y is barely winning or even losing in the precincts that should be his base, it is pretty clear that candidate X will win the election.  Even in close race, at some point, barring unexpectedly high turnout in remaining precincts, there are not enough large good precincts left for candidate X to overcome the margin.  

Now it's possible that the media call can be wrong if its too early.  We saw this in Florida in 2000 when the media first called it for Gore, then called it for Bush, then moved it to undecided.  But the media responded to Florida by putting in place stricter standards for making a call that postpones the call in close races.  But in a race that is ultimately won by a margin of 10% or more, the winner is pretty clear from the exit polls and a small number of returns confirming the accuracy of those exit polls.  

Because the media like to be first with the news and beat the competition.  It's a risky business, though, and unwise unless so much of the state has been counted that whoever's in front can't possibly be caught.  It's purely a media thing and not an official count.

It's only even possible because precincts are counted separately so the result for a state builds up bit by bit.

I think of the British equivalent - here in the UK, it's not possible to do it for individual constituencies (electoral districts that elect one MP each) because they're counted all at once.  So nobody ever "calls" those.  All the votes for Little Puddleton are brought together in one hall and counted there, so nobody has any idea who Little Puddleton's next MP can be until the counting staff have completely finished.

For the country as a whole, it's possible to say Party A has won before every vote has been counted, especially if they're a long way ahead on the results so far, but no reputable news channel would ever say so until it's mathematically impossible for any other party to get most MPs.

They do exit polls, but they're always careful to say this is only a poll, especially as they've been bitten badly sometimes by those turning out to be wrong.  Polls tend to be more accurate the bigger the win is going to be.  We do even get the BBC having an expert pollster on who will say, quite correctly, this is a close result and it's within the margin of sampling error so this poll doesn't tell us anything.  One poll they DID get spectacularly right was for last December's general election, and I'm sure that was because the Conservatives won by a landslide, so it was virtually impossible for any poll to get THAT wrong.

It's just the media trying to give the people what they want--quick and easy answers.  As long as they wait until the polls close, then I don't see a big problem with it.  The media is often wrong about all sorts of things, and yet the world still manages to go on.

Because networks want to outdo other networks by calling states first because it attracts viewers.  Therefore, they are willing to call states based on partial results and exit polling data.  

Have you ever heard the term "mathematically eliminated" in sports?  It's basically a measure of the number of games remaining against the "win/loss" advantage held by the leaders. 

Traditionally, calling election results was based on the same principle.  Once a certain percentage of the population was counted and it was determined that the results would not change even if 100% of the remaining voters voted against the leader, that election could be considered unofficially "called."

As media outlets have become more profit driven and designed to appeal more to popular narratives and story lines, however, there has been a shift away from accuracy in favor of being able to be the first to "call" out the winner.  I recall in 2012, some states were being "called" with less than 20% of the votes counted because they assumed certain districts would lean one way or another.  The fact they were more often than not correct still doesn't change the fact that they used assumption over hard facts.

Because there comes a point where it could not change the outcome even if all the remaining votes were for the other person.

The media calls a state prior to all the votes being counted.  They use a combination of exit-polls and known-counts to extrapolate a winner.  With computers crunching the numbers, they are most often correct.  Personally I think it should be outlawed.