They won't. It's a team game, and just with any team game, you win or lose as a team.
Even if it hurts losing LP when an ally disconnects, remember that you've also won a larger proportion of games from enemy AFKs and disconnects. Assuming you don’t have any connection issues or bugs, the likelihood of having an afk/disconnect on your team is (proportion of players experiencing connection issues) multiplied by four (for every player on your team excluding you)
The likelihood of having an afk/disconnect on the enemy team is (proportion of players experiencing disconnection issues) multiplied by 5.
In other words, if you take into account every game you’ve had with league, every 4 games you have an afk/disconnect on your team, you have had 5 victories owed to disconnects/afks on enemy teams.
So even though it sucks to have an AFK on your team in the moment, you've won a good amount of games because of them too without knowing it. So I'd take it in stride