why does there seem to be so many rookie starters in the NFL this year?

Because they beat out the veterans in training camp and the pre-season.

There have been a lot of injuries and players retiring in the offseason this year, and with the gaining popularity of the two running back system, starting a rookie RB carries less risk with potential rewards, so it has become somewhat of a norm, it seems. There has also been a lack of talent in some positions across the league, such as QB or RB, and teams are hoping to fill those holes with the incoming players.

EDIT:

To Sundi (below):

Retired(or unsigned) players... McNair, Culpepper, Strahan, Rod Smith, Sean Alexander.... They were all starters in 2007, and I'm sure there are a few that I am missing.

Injuries.... Umenyora, Merriman, Boller, T. Smith, Kevin Curtis, Bobby Engram, Willis McGahee, Deion Branch, Angelo Crowell, Jerry Porter, Napoleon Harris, Madieu Williams.... and the list goes on. All starters.

These type of player losses, coupled with talented incoming rookies and less talented veterans, is why we have seen so many rookies this season. As far as rookies winning the starting job over veteran stars, that hasn't really been happening more this year than any other year. There have been some, such as Matt Forte, who have filled a spot where a team lacked real talent, but we haven't seen any big names get replaced.

As for Warner, and your other complaints, why would you not start the player who you feel is more capable and talented, rookie or not? If teams stuck with their veterans over the younger guys, simply based on experience, the nfl would be a bunch of washed up guys from the 90s, and not the competitive environment that it is today. Besides, you act like Warner was some superstar. Beyond maybe a few brief flashes of above average talent , he is and has been mediocre throughout his entire NFL career.

AG- there hasn't been a lot of retiring- the older players are just losing their starting positions. Which kinda sucks- in my opinion it sucks the most at the QB position how this has been happening year after year.

You have QBs that lead a team to the playoffs many years or conference championships or even the super bowl and here they are playing as backups. It's crazy. An example is Kurt Warner. First of all, his story was already amazing how he came from the Arena League but its crazy how he ended up having to fight for a starting job for the past few seasons (he actually lost the battle the last two or three).

Also, there are many rookie starters because they are considered the most talented person on the team for the position. There are players on the team with more experience but obviously not as much talent

teams are building futures.