Not sure.
I've wondered the same about Donald Duck.
He'll never wear pants, but when he gets out of the shower, he'll be wearing a towel around his waist...
Related Q&A：
Why does squidward have no pants？-Well, you know, in the wacky world of SpongeBob SquarePants, Squidward's lack of pants is just part of the crazy and unique character design. Maybe it's a way to make him stand out and add to the zany charm of the show. I mean, think about it, in this underwater cartoon universe, the rules are different. It's all about having fun and being creative, and not worrying too much about traditional clothing norms. So, that's probably why Squidward goes around without pants. Oh, and it sure gives us something to laugh about!
