I do see it, but rarely. But not because it's not there.
Like the above poster said most NFL teams play in domes or astro turf fields. Like in my Pats chise, there are only a couple of times in a whole season where I am playing on real grass. (Gillette stadium is turf)
But if you're playing on real grass on a rainy day, you do see it.
NCAA teams play on real grass more often. So it's natural to see dirty uniforms more often in real life and in the video game in the NCAA. And less often in real life and in Madden2010 in the NFL.