The AI isn't smart, like Primal said. Also, I think they spam Acupressure to boost your stats.
Acupressure chooses one of the target's stats at random and raises it by two stages. It can raise either the target's Attack, Defense, Special Attack, Special Defense, Speed, accuracy or evasion stats. (Via Bulbapedia)
It can be nice to have your stats boosted to help defeat the Dynamax Pokemon, but it is way too gimmicky to be actually useful. Game Freak thought Acupressure would be helpful, but it can hinder you at times, or be useless. These are the only reasons I can think of. I hope I've helped!
Sources: Bulbapedia and experience