ZOS wants you to continue doing the same content over and over again. Heck players have asked for a token system where when you complete content you get a token. That token can be used to get the armor or weapon piece you want. Of course that will never happen because then folks would not have to continually do the trials, pledges, vDSA or Malestorm farm for the items they want. Players have also asked that all items players get from content completed or found can be sold but that will not happen either. Again folks would not have to play the farm game they could just buy the items.
70 keys and counting for earthgore shoulders in light or heavy, didn't need just wanted more mix and match choices, pulled 1 shoulder in medium which i had already(key 65ish). Pulled over 30 Vykosa. Sometimes I'd get 3 Vykosa in a row.
Speaking of loot. lets talk about other things.
PvP elite weapon boxes. Will pull 4 marksman bows out of 5 boxes. I've had as many of thirty of these at one time sitting in my bank.
Rewards of the worthy. 80 percent of the time its a bow, shoulder, shield, or helm. Usually Steadfast or Battalion shoulders and helms. Anguish and Acrobat for the shields.
Still seen only 3 Automaton dagger drops in 10 months.
Had 7 SPC resto drops in 2 days. Only seen 1h once and that was a dagger.
If you don't need it or it has no value thats the drop you will get. Every single time.
While RNG can explain the difficulty getting some things, RNG would largely allow a player to collect all of the things in a reasonable time.
I suspect that certain pieces are more difficult on different accounts, just so that everyone feels a little pain.
If this "pain" wasn't built into the loot system, 99.9% of players would have collected everything within 200-300 keys. Some of these bad luck streaks reported by players have a "miniscule" chance of happening if it was truly RNG. And if the pain points were the same in every account, then people would quickly identify the pieces with a very low drop rate. (And there wouldn't be a normal distribution of gear in game.)