Well, either modern matchmaking algorithms are absolute dogshit at doing this, or there just aren't enough low-skill players out there to be matched with. Because if you're just a casual player, you won't be losing 50% of games, you'll be losing 100% of games. Even at the lowest of the lowest tiers, you'll still be losing quickly and often if you don't put sweat into it.

It’s mathematically not possible to maintain a 50% win rate across any group of players unless everyone has exactly the same skill level. The proof is complicated but the idea works like this:

• assume not everyone has the same skill level
• ignoring duplicates, there exists a player with the highest skill level in the group
• the highest skill level player always gets matched with players of lower skill level, winning >50% of their games as a result
• the players who play vs the highest skill player end up with less than 50% win rate, so the system gives them more matches against lower skill players to bring it up to 50%
• these lower players then have too many losses, so match them more against even lower players
• repeat the above process like dominos falling (mathematical induction) until you reach the lowest skill level player
• the lowest skill level player has no one lower to match against, so they cannot reach a 50% win rate, thus there is at least one player with below 50% win rate

So there you have it. In practice, matchmaking systems need to compromise on the skill they match people with if they can’t find enough players of the appropriate skill level. This results in a wider range of skill levels ending up in the same game when not many players are online. This can result in even more players ending up above or below 50% win rate, depending on where they stand skill wise.