Because games are made to feel "fair" and prevent abuses.
I've read a blog about certain popular strategy game. When you're fighting there, you win chance % displayed. The game would cheat, however - if you lost many 50/50 games in a row, next one was guaranteed win. if you had 99% win chance, you always won, no exceptions. It's just that they wanted players to feel fair, and not screwed by RNG.
When you are playing competetive and player leaves, you still have the same team. You're at disadvantage, but you can win.
Now if someone leaves and other player joines the match, and you lose - he will feel that it's unfair. He joined already lost match. Maybe team was already low on morale.
If someone leaves and other player joins the match, and turns out to be really good and his team win - enemies feel it's unfair - bad player got replaced by a good one.
It's also impossible to say how much the player counted towards the win. The team might have won without him, and he wasn't much of a help. He might have played really badly and made team lose. He might have carried the team which was otherwise bad.
Balanced matchmaking is already hard enough with all these additional complex factors.