Because it's not a fucking issue. Look at him play. He held is own fine against a team like Atleti. At no point have I watched him play and complained about his lack of muscle.
You think these mesomorphic players look ripped just because they need to wrestle with opponents? Of course not, it's mostly about movement, look at sprinters.
If you're a Benz, Kane, Suarez type of attacker, then obviously you need to wrestle, but Puig is not. He works around opponents, not through them. Wrestling is like 5% of his game and it's not like he just falls down. He actually seems to have great balance and rigid core. Might get pushed back a bit easily, but can fight back by slipping by and poking at the ball.
He's nimble like a fly. Try killing a fly by pushing it away.
The match against Atletico is good but its just one game. Watching other matches opponents do outmuscle him quite easy when he defend against them. You can watch his U21 game and see how easy he is knocked to the ground by his opponents.