It’s probably because the different “forms” of Sawsbuck doesn’t have different movepools than each other.
Like Lycanroc, and Meowstic. They have different movepools and abilities (at least Meowstic does) from each forme, so they get different tabs, and like Kyogre Pulse mentioned, it would take a lot of work to add in ALL of the Pokémon with different formes. I get it that you would add them in if they have different movepools along with different formes, but you’re just putting in more work if you add the tabs for no other reason than just different looks.
Now, I don’t know for sure, I am no expert and don’t know what’s going on inside Pokemaster’s head, so I don’t know if they didn’t do it for a different reason. I am just putting this in because it’s just a theory that I have, and it makes a lot of sense