Uh yeah because it's a movie. They couldn't pass up on casting Kermit the Frog and Miss Piggy as Bob and Emily Cratchet (what with Miss Piggy's sheer talent and star power) which left them in a bit of a casting bind. Realistically they had to cast pigs and frogs as the Cratchet children instead of searching high and low for talented half-pig half-frog child actors. Even then they went out of their way in casting Kermit's actual nephew Robin the Frog as Tiny Tim and the other child actors (Peter, Miranda, and Melinda) pretty closely resembled their 'parents'.

Which only mean they aren't a frog nor a pig, they're an entirely new species where the male have a frog-like form and female have a pig-like form.

First off, Hank Green, it's The Muppet Christmas Carol. Second, it is the best film version of A Christmas Carol hands down. I will die on this hill. Finally, we pretty much kicked any scientific accuracy to the curb, starting with pretending that Gonzo is Charles Dickens.

May be they couldn't have kids because, i don't know, being entirely different species, so they adopted.

It's like how in the Shrek universe, dragons and donkeys are the same species.

That’s better than those demon dragon-donkey hybrids from Shrek.

Sometimes this happens with dogs though doesn’t it. Maybe they just took it to the extreme for puppets

Of course it's Hank Green who would question it lol. But you're not the only one!