Agreed. In terms of actually injecting into a muscle - yep anywhere. BUT...
There are good points or sites to use. So some have found for themselves that an inch off the perfect spot and OUCH THAT FKIN HURTS cos that's where a nerve is. Ditto, from trial and error, veins etc can be pricked by a needle. There are online guides which show the perfect spot.
The main reason, other than just swapping sites about (the glutes are No 1 but it can, quite literally, become a pain in the butt) is so-called 'site-injection' with the idea that the steroid will, somehow, have a localized effect. While there's SOME evidence to support this, the effect is minimal. As in not really worth the effort.
Now we all have seen the site injection oils (SEO's) crazy mofos. Some, lord help them, have risked amputation by using (so rumor has it) vegetable oils and worse (inc - fuck me - motor oils). The original idea was a form of self correction of an imbalance. So injecting into the outer head of a bicep and so on. But it's too easy to see how the typical user fucks that up and, assuming it wasn't taken to an extreme, it's supposed to be absorbed back into the body anyway. So what happens to the correction? It fkin disappears. Others claimed to fill in with oil and have the muscle grow into the space created (this includes so-called fasica stretching). Even those that kinda got this right seem to produce a weird looking (by way of samples taken) muscle which is not quite normal.
Finally, so he claims, we have the in-famous Greg Valentino (he's kinda cool). He claimed to inject such large amounts of actual steroids or added more of the carrier to a steroid that it was this that caused what he had. Not, as has been said a LOT, that he was injecting SEO's. I'll include UK based roid guru Dave Crosland in this. He has a DVD documentary of him jabbing three to four roids (at the time he was on 5000mg a week) and it was tricep, delt, calf and so on. That many mg is a LOT of ML's of fluid a week.
Most of us, especially the more conservative users, simply don't inject the sheer amounts of roids and fluids required to make it worthwhile. And, as per another roid guru buddy of mine used to say: If site injecting worked and we all jabbed our butts then how come we don't all have giant butts?