Why didn't Liz want a wish?

Because she's a classy lady and doesn't really care about all that "boys own" stuff...

I took it as her being content with Bishop getting his freedom... and by extension she gets Bishop...

When you're as sexy as Mary McDonnell was in this movie you don't have to wish for anything because you can get anything when you want it.

'Cause I'm Black you think I did it?

Well...

Even though Liz was "in" on the gag and her safety had been endangered, she was not really "part of the group" who earlier in the movie suggested what they would want (in Liz's presence).

Full circle: Each person asked for the same thing at the end of the movie as they had each asked (fantasized about) earlier in the movie.

As another poster has alluded, Liz was not part of the boy's club and, therefore, not entitled to "payment/consideration."

I always thought her declining was one of the funniest scenes in the film. Great movie, very underrated and, I think, a bit ahead of its time.