Riot made that a thing back in season 7, and I can see a reason to it, but I don't think it should be a thing.
The reason I can see is so that the people who have to AFK for something real-world in champ select can't do that and still get into the game. Why? Because before, you could hover a champion, get called away by a parent to do something, your champ would lock itself in at your turn, and when you got back, it's already 10 minutes into the game and now you have to load back in.
However, this doesn't really prevent that as much, and more makes it so that I have to wait an extra 20 seconds before I can go use the bathroom before a game. It's less of an AFK deterrent and more of a minor inconvenience.