Why do two baseball teams face each other 2 or 3 times in as many days? Like shouldn't the results be decided after a single encounter?

Why do cricket matches last 5 days?

This year is different of course due to Covid, but normally, the regular season is 162 games and lasts 6 months--or about 26 weeks.   In other words,  teams will play about 6 games a week.  It would be way too hard to do that if they had to get on a plane after each game.  That's why they'll play 3 or 4 games against the same team at a time.

Because that's whats realistically possible in a 162 game season. Travel costs and the time required for the season would be prohibitive if teams only played 1 game in a location and then moved on. Sports that play one game and then move on have much fewer games in a season.

Why keep track of individual games at all? Just keep a running tally of runs scored and runs allowed and the teams with the largest run differential at the end of the season are the ones who make the playoffs.

Then have all of those teams play each other in the postseason and the one with the largest run differential is champ.

In the event of a tie those teams play each other to determine the champion.

Because baseball games don't go on for three to five days like cricket matches.  

Just as cricket is played in "tests" (I believe that is the correct word for the full, multi-day match), baseball plays in "series," most of which are either 3 or 4 games. Both the "test" and "series" formats allow teams to involve their full rosters and gauge how their full teams "match up" against one another.

Regular season records are based on individual game results, but analysts will point to series results as the better gauge of a team's overall quality. This is because, in the playoff rounds, champions are determined by series results.  The regular season, therefore, is like a preparation for the playoffs, allowing teams to see how they match up over multiple days.

That way they can have more games without as much travel time.