Because a ban/restriction is based on a pattern of toxic behavior over many games. What you propose is a punishment for the unreported incidents during a single match. It is actually very different.
Instead of forcing a chat restriction mid-game on the player, individual players can make the choice to mute them. Anyone that doesn't have a problem doesn't have to mute, and the person is not arbitrarily punished.
Now, there is still something between those two; actually reporting a player in the middle of the match. Doing this could result in a chat restriction mid-game. However, the problem with that is people are taking time out of their game to submit reports instead of focusing on the game, which is not something that should ever be promoted. The mute button that currently exists and reporting them after the match is the only reasonable option.