We mute a player in-game because we don't want their chat, so why do they get un-muted in post-game?
I'm serious, it's really simple, we probably don't want to see their chat. We didn't like their attitude, or they were harassing others / were annoying, or other similar reasons. If I muted them before, then there's few - if any - cases where I'd actually care to see their chat in the post-game.
Riot Pls. Make mutes carry over through pregame, in-game, and post-game.
Edit: Another user came up with a better suggestion in the comments, see here.
I think muting is supposed to be a temporary thing. This means that you can use it to tidy up your chat by muting a nice but chatty person or even a friend you queued with. If you don't want to see any further content from someone, including lobbies, in-game, etc., that's the "block" feature.
Rather than automatically escalating every mute into a block, how about allowing players to use the block feature while in a match?