>>39787848They have unconventional bulk, Snorlax is blubbery and Muk's body isn't even solid. Ash never ordered them to tank hits, they just did out of instinct.
Basically, here's Ash reacting to an incoming attack:
"Oh no, an attack! Dodge it!"
"Counter it with your own attack!"
He never says:
"You can take the hit! Strike right after with full force!"
"You'll resist that, push through it!"
And this is because he's not an actual trainer, he barely knows his pokemon's types and moves. His priority is the pokemon's feelings, he's a pet owner at best.