Not everybody is on HRT because of AS abuse. Me for example (according to my doctor who knows about my prior AS use).
Men's testosterone levels start to drop in the late 20's/early 30's and by the time they reach 40, are drastically different than when they were 18.
There is a host of health issues associated with low T levels.
You can either live with it, or you can supplement your T levels.
As an aside, most don't need the shot to screw; for some with libido problems it helps, but for me, it's mostly helpful at maintaining mood and keeping a healthier body that supports more muscle.