I justify those movies as being made for 17 years olds (naive, immature, mush for brains...). Knocked Up here was rated 14 and up, fyi. My tastes have less and less in common with teenagers the older I get (which is surely a good thing, as teens don't have very discriminating taste)
I don't expect much from those kinds of comedies, so I rarely waste money on them. Having seen Talledega Nights for free on cable, it had its moments. Anchorman I couldn't get into so I've only seen 10 minutes of it.
Same applies to most horror and "super natural" movies they pump out. Most are aimed at a rather unsophisticated crowd (which is why Rob Zombie can get away with directing movies and having them actually get distributed to theatres). Me, I'm too jaded now. All those movies are predictable, as I've seen the same plot lines or jokes over and over before. I Am Legend was good and I would recommend it, but even that one is essentially just a well-polished mash-up of Omega Man + 28 Days Later