He's not a striker, that's why.
He's a wide forward like Salah or Ronaldo who is an excellent finisher.
He can do a job upfront against the right opposition - one that let's him use his skills from out wide against the defence like running off the shoulder.
Thats not being a striker though, and he lacks all the other skills. He can't hold the ball up and play with his back to goal, his movement when against a deep defence is non existent and he doesn't drop deep to help the midfield and link up the wingers. He cant get tight to a centreback and pull them out of position.
Lacking these skills makes him a passenger in a lot of games and go 10 minutes without a touch, which is why Pep didn't play him last week in 3 massive games where we couldn't afford that.
If people looked at the role of striker as a whole instead of just thinking "ooh he scores goals, must be a 9" then they'd never think Torres is going to be a striker, he lacks 80% of the skills that the position requires.
Will he learn them? Probably not, it would take years of playing there every week. Could he be a great wide forward and be rotated into false 9 against the right sides? Definitely.
The key for Torres is the Gabriel Jesus partnership because Jesus does the 80% of the striker role that Torres can't, and Torres has the clinical finishing Jesus lacks.