In order to bring The Simpsons together with other TV characters, one must create another suspension of disbelief on top of others already needed to accept this show’s non-aging characters across three decades.
(Granted, Bart and Lisa’s hair-skulls should weird everyone out already, but I digress.)
And in most cases, having more traditional-looking humans such as King of the Hill’s core family in Springfield just makes the town’s characters stand out in weird ways.
It might not seem like that big of a deal to fans who aren’t thinking about such things and just want to see Rick and Morty and The Simpsons getting a full-length crossover, but the writers in charge of the characters and storylines do adhere to a specific kind of logic.