In the case of Project Hail Mary they have an "answer" as to why. The book directly explains it and the movie nods at it in a scene where he writes "Muscles?" on the white board, but its the coma robot, Armindo. It was meant to work their limbs and apply electrical stimulation so they wouldn't atrophy on the trip.

So, I was watching 'The Getaway' with Steve McQueen. That movie was made when McQueen was one of the top box office draws in the world. He takes off his shirt and, to me, he looks a little scrawny.

A while later, I'm watching a movie where the shirtless guy is supposed to be a middle-aged lawyer with a minor drug problem. Looks like he could throw McQueen through the wall.