Unlike other comics like Shencomix which uses an author self-insert, and Blob and Friends which are a bunch of OCs no one really cares about, Strange Planet's aliens allow ANYONE to self insert themselves into the comic.

The comic's jokes can replay every single joke in history just by using synonyms, it's so easy to make material.

And recognizeable merchandising is profitable and easy! There is literally only one character. They are all the same character. And no one cares.
That's just maximum mascot recognition. Like if there was a pokemon game with only pikachu, not diluted with the unpopular pokemon.