This is answer number 1. The guy from the Simpsons asks you do to voice work, which I have to imagine isn't as much of a pain in the ass as being filmed.

I think answer 1b is they are also getting paid for said work on a show by a legendary creator.

1c is they are usually themselves, which means there's no confusion as to who the person voicing the character is. It's great for their brand.

Not to mention Futurama has its own high profile people that other celebrities would probably enjoy working with, and probably had their own pull.

Also, they originally aired on 20th Century Fox, which seems to have a group of famous people that will pop on to random shows for cameos. I also think it’s easier for celebrities to do cameos as voice overs, since they can be recorded at any studio - even home studios.