You, good sir, don't understand how matchmaking systems work.
Basically a matchmaking system is trying to get 10 players into a game so they can play. If there are no restrictions it can do this instantly because there's always enough Bronze players to fill out a group of 10 for that 1 Diamond player in queue.
However, just getting thrown in with anyone sucks so you want to add restrictions to the matchmaking formula. Every restriction comes at a cost of longer queue times, however. Why?
Look at it like this: If we have no restrictions we can get instant queues because there's always at least 10 people wanting to play the game. Now we add a restriction that all 10 players must be within 1 rank of each other. Now queue time takes longer because there may not be that many players close to you queueing up right now. Wait a bit and they may show up.
So now we add another consideration. Everybody gets to pick their roll. Now you can't just grab any 10 people from the same skill level, you need the right 10 people. No supports? Well looks like you're waiting until two of them who are your same skill level show up.
But you say, premades bring in more supports? They might, who knows, but what they definitely do is add another thing the system is trying to match. You've got a three man in queue and seven solos so nobody gets a game until another three man of the same skill level and seven more solos come in and all those new players better be exactly the one's needed to fill out the rolls that are lacking...
"But," you say, "if matchmaking worked like that it would take forever to get in a game!" True, I simplified a bit. The longer you're in queue the greater the variance the queue will accept in each category. Been waiting for two minutes? Well now players can be two ranked apart. Been waiting for five minutes? Now your five man can fight five solo players. These matches won't be as fair but it's better then waiting for a hour to play a 40 minute game.
Premades are particularly bad for queue times and match quality because they introduce elements that are much harder for the matchmaking algorithm to work with. Firstly you cannot ever put two three man premades on the same team. It's obvious but it means there is no amount of time and sloppy matchmaking that can build a team from certain combinations of players in queue. Second, it's much harder to properly gage the skill/ranking of a player in a group then it is a lone player. Some premades are just a bunch of random people who play together but don't use VoIP or any other advantage. Their no better then a group of solos. Other groups are coordinated and ruthless with persuasion rotations and players filling in for each others weaknesses. These groups perform much much better then five randoms. If the matchmaking formula assumes premades will be like the former then the later get to stomp every game while assuming the later advantage means the former gets their face caved in every match. So most likely you assume some level of advantage in the middle and both effects happen to a lesser degree. Yah, such balanced games!
Even if premades provided enough supports that they completely balanced out the other roles (they don't) the system would still need to make teams of 1 of each roll and account for premades which will take longer then just accounting for 1 of each roll.