From my understanding, this is intentional, rather than one of those "SE just ignored it and didn't notice it was a problem" type things. Because pre-formed parties are significantly better and more coordinated than solo queuers to the point the others probably don't have a chance.

There could be a separate one that is only for partially pre-formed parties to queue for. I don't see a reason for them not to do that. Then it's pre-formed parties vs pre-formed parties. It just would be impossible to match them if the party was 4 so it would have to require either 1-3, or 5, unless a solo person was willing to face lots of organized parties on purpose.