ok, first, you do have to do the relic quests over completely for each relic. you dont have to do it once and get a free pass for every other relic you want to make.

second, people do low level roulette, which will put them in any low level dungeon(obviously). i queue for that as tank, healer, or dps, depending on what i feel like.

you cant force someone to build up every class and force them to do every story related thing there is. i only play the level related dungeons for the class im building. people dont fate grind completely to 50 like you seem to think they do. if you play at night, youll be lucky to get anything, ive waited 40 minutes for a dps, we made it from the first boss all the way to the end of the dungeon. nothing is broken, its just that you feel like you should be catered to. "oh, it takes forever to get in a dungeon and sometimes you cant even get replacements, so everyone should be forced to do low level stuff and level up other classes".

i join in progress for everything, and sometimes i will withdraw if i dont get on already in progress. you just have bad luck.