That's not a function of the Pokemon, though. That's a function of the Pokeballs.
In theory, you could capture humans in Pokeballs. All they're really doing is digitizing a life-form that can be stored as computer data (sort of like how Flynn gets zapped in Tron). And the anime has shown other things being captured (like inanimate objects), so there's nothing inherent in Pokemon themselves that is a necessary part of the process.
The implication is that Pokeballs are probably designed with safeguards to prevent targeting humans specifically, for obvious reasons. But you could probably design/create one that could (though it would likely be extremely illegal in-universe).
But also, we're talking about a franchise made for children. I've probably put more thought into the concept in this post alone than anyone actually working on the game did, because they didn't really care.
Professor Laventon from Pokemon Legends Arceus "What's so strange about them, you ask? Well, each and every Pokmon is able to shrink itself down to minuscule size!" "And that's where Pok Balls come in! Pok Balls are a recent invention, you see. Throw one of them at a Pokmon, and...why, the Pokmon will shrink down and fit inside the ball, comfy as can be. And with that, you've caught a Pokmon!"