This post is interesting and probably still relevant:
If I understand it correctly, the matchmaker tries to mirror teams in terms of group size. So if, say, I’m solo queueing, and the enemy team has a 4 stack, then it’s very likely that the other 4 people on my team are in a group. Overall, such a match is technically fair, in that both teams have equal-sized groups. However, in my opinion, the subjective experience of the solo queuer isn’t great. Not only are you fighting at least 4 enemies that are well coordinated, but you’re also “othered” by your own teammates because you’re not one of them.
I feel like blizzard should add a solo queue option. At least that way everyone in that queue is in the same boat on both sides.
That would be neat and I’d probably use it, but I doubt they’d want to split the playerbase like this.