The easiest way to prove this is to see who on a film DOESN'T get a credit. It's going to be non-union positions to go uncredited. For as many credits as you see there are hundreds of unlisted ones. Just about anytime you see a company credited for vfx and you see only 10 names, an easy 20 were left off.
It cost money to add credits to a movie. Studios don't do much more than they have to.
I got a credit once. It was cool and made my mom happy.
Thanks kind stranger. That was nearly 20 years ago for me, but still neat.