Pokémon Let's Go - Eevee looks completely exasperated

That was my reaction when I learned it couldn't evolve

Yeah, I don't think they thought it through enough. I can only imagine the meeting that lead to this idea:

"You know whats would be cool? If the Pokemon whose main point is to evolve into like 8 different things wasn't able to evolve"

"Ah, designer-san, that's a great idea. You've done it again, you absolute madman."