We get it. You're a monty-haul GM who can't accept that the experiences of the rest of use 30+ year veterans don't match yours.
You're the outlier, by the way. Which tends to imply you're wrong.
The only times in AD&D that I've seen thieves doing lots of damage was when they were multiclassed fighter-thieves. Using a d10 or d12 weapon with a x5 backstab was always a big hit, but it's also not a pure thief role.
Many GM's didn't give away magic items like candy. (I did, for a while. I got over that real quick.)
Many GM's didn't let thieves get more than 1 backstab attempt, and even then, required a check on move silently.
My group is hardly the outlier. I've played with a ton of groups over the years. Our group is about the middle for magic items.
Getting a girdle of giant strength at the higher levels wasn't hard. I gave the rogue a fire giant girdle because the fighter or other martial usually handed it down after he got topped out. It was fairly common for a bunch of kids playing D&D back in the day to want to have nice magic items. Maybe you were already an old man or played with a particularly mature group, but stop pretending that all of the young D&D players of that day somehow had the sense to limit magic items and the like. It certainly wasn't the majority as you claim. I went to many a table where our magic item selection appeared meager because we usually handed out what was in modules.
Our group, ,like many others, was a bunch of young kids playing D&D in our teens. In the course of having fun, we liked getting magic items. Those old modules which you seem to have forgotten, handed out quite a few, including girdles and +2 weapons.
I am indeed talking about higher level thieves. They had it tough at lower levels.