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?
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.