It used to be like that, and it caused hosts to kick impostors without them getting voted by majority and a lot of people found that annoying. They wanted to at least defend themselves but hosts would just kick them and that's that.
So this measure was implemented to prevent that. If people collectively agree to kick someone out mid-game at least it is done with the consent of others and not just the host. That way a fair amount of people will accept the decision being made. If the host just did whatever they wanted it would spoil the experience of the game.
Kicking and banning after the game is different because you can now decided to keep them for the next game or not and it also allows *new* people to join before the start of the game. I think getting new players into the lobby while removing those you no longer want is what makes the difference here. As you can't replace people mid-game.
When you kick a crewmate mid-game the game removes their share of tasks therefore lowers the total number of tasks among all crewmate members and therefore tilting the favor towards the crewmates. How? The less tasks needed to be completed the quicker they can end the game before the impostor killed all of them. This can be the same with disconnecting mid-game. But in most cases this is a server-sided issue or just an issue with connection between the player and the server depending on wifi strength in general.
When you play 1 impostor games, the host doesn't need to convince people to vote the impostor they can just kick the impostor and win the game, NOW this is not how among us should be played... EVER! And this was basically what people were doing when hosts had the power to kick people mid-game without anyone's consent.
In a 2 impostor game, the host could do the same thing, except the only difference is the game doesn't end until both impostors are out. With 2 impostors, kicking the first impostor you find prevents them from being a ghost to help with other impostor. so this tilts the favor towards the crewmates. This more or less applies with 3 impostors as well.