Because they've never been called a different species in the show. That's the essential fact. Not like demons, or monsters, or pony heads. Just a different last name.
Their appearance is no different than different types of Mewmans either. In fact, since Star has cheek marks and wings you could argue she's not Mewman at all either, just a Butterfly, yet no one's doing that.
Different skills can be attributed to the Buterflys having magic and Johansens, like other Mewmans, not having those abilities.
Also with Star taking so much pride in Mewnipendence Day, making an effort to celebrate it even on Earth, calling herself Mewman, etc., you'd think her supposedly being half-caste and not full Mewman would be an issue and yet it's not.
If we don't have a different species name given, then they're probably meant to be the same.>>91214530
Also there's obviously some class distinction/family rivalry/whatever going on but with a theme of this show being species genocide/discrimination you'd think it'd be spelled out if the Johansens weren't Mewman.