Hasn't happend lately, but I know what you mean. There was some two day periods here and there were there was always leavers in the match.
Probably server instability mixed with the "cannot reconnect to game" bugs.
If someone's been offline for a few minutes the game should let you surrender. Winning a 4v5 is so much harder in hots than in other mobas. You can't really carry your team to make up for the missing player.
If you're playing 5v5, you can still win the game with a good teamfight no matter how badly you are getting crushed. Just try to survive until people are in the 18+ level range so the res timers are long enough for you to make stuff happen during their downtime. If you can't survive up to that point, the game only takes like 15 minutes in which case I don't see much benefit from surrendering.
Definitely do not want a Surrender option >< I've had an absolute blast even in games I've lost, and that's what games are supposed to be about. If you're the kind of player who doesn't get that, you probably shouldn't be playing competitive games in the first place!