Every time we have a shitty or subpar year, we wonder why the coaches don't just throw it all out the window and become wild banshee play callers.
And it never happens. In fact, it seems like they more often become even more conservative than they usually are.
I think for one, coaches realize that their team sucks and the only real chance they have to winning more games is to protect the ball on offense and put the defense in a position to not give up too many big plays. This way we can take advantage of field position to get a field goal or a few touchdowns while the defense can wait for the opposition's offense to make a mistake. Further, it kills the clock keeping everyone rested.
Secondly, after seeing the ineptitude of our players in executing the simple plays, you really want to watch them try to do something complicated?