Because at this point most people have been in game long enought to have run each and every dungen/trial/raid multiple times. Roulettes and things like relics force us into old content over and over again. Of course at some point you'll remember it.
But don't be afraid to run dungeons, just cause you feel you don't remember it. Mechanics are mostly easily figured out on the fly in normal dungeon runs and even if you screw them up, they most likely won't wipe the party. Also, as long as people see you're trying and giving your best, they will not give you a hard time over not dealing with something correctly. Sure, it does happen you run into such players, but luckily I feel the community isn't that bad and such encounters are rare.
Anyway, keep running your roulettes, you too will remember the fights after having seen them a couple times, if you're really feeling insecure don't be shy, tell the other party members you've only been in there once, have forgotten about the boss and ask them to explain it to you.
Hope this helped a little, enjoy the game and have fun running those roulettes