Mostly because they ball against a bunch of other guys you'll never see or hear from again. It's the practice reps that matter the most because that's when the guys that are making it are getting reps against stars and not a bunch of other guys that are going to be out of the league soon. It's not fool proof of course, and there are certainly things you can take away from PS games, but they definitely matter much less than they used to.
Most of the time, if you get a rotational guy at the back end of the roster out of a UDFA class, you're doing well. If you get a low level starter, you're doing great. If you get anything more than that, count yourself lucky.
if you're making a roster on the back of PS games it's got to be a ridiculous set of performances... e.g. michael pierce, keyon martin (last year)