I’ve been going back and watching earlier seasons, which - I thought - had a smaller budget and less recognition. But after taking a closer look at the credits I realized there’s a lot of celebrity heads that are voiced by the actual celebrities. The star trek episode actually had the full original cast - and those weren’t bit parts! I get that the show’s a social commentary, but wouldn’t it have been easier to get sound alikes? Is there a story behind the show’s giant special appearances list?
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.