I think matchmaking has a lot of competing priorities, and keeping players together is low on the list. In some cases, it would be impossible. Sometimes people search for specific maps and game modes, and they might be with you when they coincide, and split next round when you move on to another map that isn't on the list they searched for. You might have joined a generic random map rotation of all-out-war, but they searched for a specific game mode and one or two specific maps. You join and then split. Just an example. There are also people in different places which next round more perfectly match with anothet lobby that was just thrown together closer to them but further from you. There are also people who just started searching for their first game if the day and others who just finished their last game of the day. When it is all a floating demand and instances of servers popping up everywhere to meet demand and performance expectations, keeping people together is low priority or even anti-productive.
I suggest you search for and join a hosted community experience. Those will keep you together round after round because they aren't using a matchmaking system prioritizing changing demand. If you don't see the experience you want to play, host your own.
We need to start loving portal and giving community creators our attention and time. It's the only thing we got that still has a lot of love and passion in it. We need to overwhelm the bot farms and toxic exploiters with love and attention of the good things and the experiences we want to support.