In principle, someone could get really good at team oriented plays and be functionally much better than their solo/duo queue rank would indicate, and vice versa, so using solo/duo rank as a queue restriction factor could result in some unfair situations as well.
It'd also be really weird for a premade 5 to be all the same flex rank and be rocking their way up flex, but then have a teammate rank up/down in solo duo and suddenly no longer be able to queue together.
It also wouldn't change how people still wouldn't take it terribly seriously, so for any given skill level, you'd get a mix of people who are actually trying, and people who are basically using flex to learn new champs and not taking it seriously.
Flex will eventually sort itself out if people play it properly for long enough (if diamond skill players are stomping random silvers, they'll eventually end up diamond flex with an associated mmr), it's more just people don't play it long enough to really get their mmr to settle at their actual skill levels, hence a lot of high skill players with relatively low flex mmrs.