The additional computing power required to run the extra code just to calculate all of the extra info needed to forcibly give people winning or losing matches would be far too expensive for a business to maintain. For a single player it wouldn't weigh on the system too much to run the few extra lines of code, but multiply that by every user concurrently logged in and playing on your region's server at any given time. That's a lot of times to re-run the same lines of code. And besides, it would double queue times because the pools of players available to matchmaking would have to be split in two (people owed wins and people owed losses).
50% win rate is the outcome of the system, a byproduct. It's not a setpoint that Riot uses. With setpoints, you can tell the system what you want, in this case, to match players so that everyone has a 50% chance of winning, but it leaves out all sorts of important factors. So it fails to be accurate, and tends to over or under estimate players' skill levels. This leads to unintended imbalances in matchmaking, aka win and loss streaks. If you're playing at your actual skill level with people similar to you, you will have a 50% win rate because your chances of winning against people of the same skill level as you are 50/50.
It's the same as a control system, the system attempts to correct for the changes in your MMR by matching you with people it thinks are at your new skill level, but because your MMR is still changing (be it from a win streak, or a loss streak), it messes up and can over/under shoot, and then wind up putting you in a match you don't belong in. Now multiply that by 10 because there are 10 different players who's MMR may or may not accurately reflect their true skill level. To be honest, I'm surprised matchmaking works at all since things can get ten times as wonky. Even when you get matched accurately at your MMR, the 9 other players might be on win/loss streaks and their MMR might be inaccurate.
Riot should be using PID control to more accurately predict people's skill level instead of whatever they're doing with MMR.
See here for an example of what your MMR would look like compared to your actual skill level. The colorful lines are examples of MMR as it changes over time, and the flat reference line is the setpoint (your new skill level / MMR) after a change has occurred. In reality your skill level is constantly changing and it won't look as flat, but the idea is to show how a system's outputs oscillate when responding to a change.
https://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/PID_varyingP.jpg/320px-PID_varyingP.jpg
