If you sit out, you are giving the NFL decision makers something to question your character, which could be detrimental to your draft status. Also, once you open Pandora's box, where does it end then? Do they just not sit out bowl games but sit out the last 1-2 games of the year once their team is out of the conference race? It sets a bad precedent.
We all understand the risks involved and while it may seem logical on the surface, i can guarantee many will see sitting as choosing to not support your teammates and putting yourself before all else.....something that doesn't go over well in NFL locker rooms and GMs. If you aren't going to play in college, are you going to play in the NFL when you are nicked? Late in the season when your team is struggling and/or out of the playoff race? Are you going to play late in games if your team is losing? Look at all the "character" guys that have been talented but run out of NFL towns.....Mike Wallace, TO, Randy Moss, etc. Mario Williams was just called out by a teammate for not trying and is going to be cut. Character is HUGE in the NFL these days.