Some of the summons are smart enough to put actual characters to shame and speak no different than a regular person, so what's with the lack of human summons? Humans are hardly any different from beasts in Naruto. It'd be useful to summon the Hokage during a dangerous moment.
Because no one nitpicks things like you.
Animals can be used as "pets" or "servents", and the same applies in naruto/boruto.
A human summoning a human, via blood contract? Come on, now. But it's possible the animals can summon the humans(reverse summoning), as long as they have signed a contract with them. Humans need the animals, more than the animals need the humans, cause the human world is an "open" world, full of dangerous humans who also summon animals. It isn't the same.
Considering the circumstances, it should be this way.