The dude likely never commanded more than 1000 guys at a time before this, and he legitimately just doesn't have the training or the temperament to be in the position of running the ENTIRE military. There's a reason you make generals and admirals your SecDef: they actually have training in things like strategy, and have enough experience to know what actually wins a war.

Guys like Hegseth, who don't really have experience in high command, think "muscle and grit and cool guys with guns" win wars. This makes him more popular with actual soldiers, but it doesn't win you wars, and his insistence on culture war bullshit over actually doing things that work is rapidly losing him the soldier support too.