Matchmaking...
This has been addressed by devs multiple times. They are aware of the problem and have a fix incoming. They can’t announce a specific date because of the forum community.
Also, matchmaking has really only existed for a couple months. In the old queues, which allowed teams vs. solos, it wasn’t really possible to tell if the system would put five solo queuers together against five solo queuers of lower rank, because that never happened.
tl;dr the issue has existed for a couple months, has been confirmed for a few weeks, and already has a fix incoming that has been confirmed by multiple dev posts on various occasions.