What's the deal with uncredited movie parts?

Last night The Wedding Singer was on TV, with Adam Sandler and Drew Barrymore.

It also clearly had Steve Buscemi and Jon Lovitz, each with one scene all about themselves, with closeups.
But I can’t spot them in the credits.

So I go to IMDB http://us.imdb.com/title/tt0120888/combined
and find there are 5 uncredited roles:
Lukey Bolland … Band member at robbies wedding (uncredited)
Steve Buscemi … David ‘Dave’ Veltri (uncredited)
Zachary David Cope … Nephew (uncredited)
Jon Lovitz … Jimmie Moore (uncredited)
Brian Posehn … Man at Table #9 (uncredited)

Now, maybe Steve and Jon were too embarassed by the weak movie, but why would the other people be uncredited?
Don’t tell me those guys are too proud!

And they can’t just have been overlooked as not important enough, since they found time to credit 8 people in makeup.

So how does that work?

Welcome to the SDMB.

This issue has been discussed many a time. There are many reasons why actors are uncredited, but usually it’s because they have taken a bit part (or a smaller part than they usually do). If they get credit for a bit part, then they have less negotiating room when they try to get their next starring contract. No official credit is better than a credit for a walk-on role.

Other reasons include:

  1. It’s a cameo role.
  2. The actor didn’t like the final result and didn’t want to be associated with it.

Well, Buscemi makes a cameo in almost every Adam Sandler movie, and the two are apparently good friends. Perhaps it was just a favor to Adam, or an inside joke between the two. Lovitz and Sandler have history of working together as well (from Saturday Night Live), so I’m sure that had something to do with it too.

In Tootsie, Bill Murray insisted on not having his name mentioned in the ad campaign because he didn’t want people to think it was a “Bill Murray” movie.