Why are so many people in plat/diamond getting matched with golds/silvers/level 30's?
My theory.
Because Riot knows there are a ton of boosted animals in high elo. They are too lazy to make a system to detect and ban them and so just assume everyone is boosted in high elo and match them with people that are lower mmr.
Even if the actual boosters get reported to support Riot doedsn't give a fuck. Their support ticket system is utter bullshit and the people that get reported often don't get punished.
Solution: get a better system for detecting boosters/scripters.