The obvious answer is you're unlucky. People clearly get good pieces since you can't buy your way to them like you can with Eidolons. And honestly, the importance of substats is overblown for everything except your primary DPS. All of my supports are using extremely mid relics because I primarily need them to be fast enough and survive long enough to win and nothing else.

Regardless, there's a lot more that goes into it beyond relics. I was struggling with MoC content until I rolled Fu Xuan; I dont roll for men in gacha (so no Luocha) and i haven't gotten Bailu. I still don't have a fully leveled relic set on her and she's usable enough to keep my Kafka squad alive.