ELI5: Why is your solo queue MMR not used to adjust your flex MMR placement

As I get into my 5th Silver I flex game of the night against at least one Platinum or Diamond level player, whilst my team is composed solely of Silver or Gold players, I wonder to myself how it is that the game thinks the road to Gold should be paved with plats.

Now I understand that flex and solo are two different queues, but how can the game think someone who plays at a Diamond or Platinum level should be placed into Silver just because "its a different queue"? Why is this okay? 300 MMR difference in teams should never be a thing that happens, yet it happens incessantly.

Is there an actual reason that the queues are treated obliviously to each other?