If there was an option to never join unfinished games, then nobody would ever join unfinished games.
Whenever somebody would leave a lobby, nobody would fill his place, and eventually the lobby would become empty until the team forfeits.
It's a required mechanic.
As far as I know, there wouldn't be anyone who would want to fill his place anyways.
This is usually what happens:
*Person leaves game*
*New person joins and notices that it's a game in progress, so he leaves*
If new players are going to leave anyways, why bother leaving in that "required" mechanic?
It only frustrates joining players.