Because he isn’t a DM. He is a ball winner who normally impacts the game higher up the pitch. His role is to break up play, protect his midfield partner and spring the counter higher up the pitch.
A defensive midfielder would sit at the base of midfield and would often operate in a single pivot protecting the back 4.
Torreira couldn’t function as a DM because he doesn’t have the positional discipline or the physical attributes to play there effectively, especially in the Premier League.