My company does not promote anyone, they have 3 or 4 people doing tasks above their pay grade but refuse to promote someone to do those tasks full time

Why do big corporations can continue to get way with this? They have a terrible dynamic, I’m tired of doing training sessions with underperforming agents and still be paid the same as them, this has been going on for 1 year and no one is promoted. Honestly, at this time, they can shove the promotion where the sun does not shine. I’m tired of this dead-end glorified customer support job.