but your personal MMR is gained by doing team content. Your personal MMR will never rise if your team sucks. It's the same as with rio.
Nobody said raider.io is a perfect tool. It simply shows how experienced a player can be expected to be. Expectations won't always match reality. Sometimes people got a higher score than they should have because they cheated the system. It's not a perfect system. No system is. But it's good enough for a quick estimation and that's all that matters. In the really high keys people might start looking at other data. But for +15s rio is good enough.