Because when you type in game you can directly and irrefutably break the rules.
When you go 0/5 and leave, you could have bad internet issues, or you had an emergency, or maybe you're just in a bad mental state and realize trying to continue would just make things even worse for your team so you leave, or you might be malicious, and more possibilities, all of them equally valid and likely.
As a result, the punishments for leaving a game are a loss even if your team wins the 4v5 which is absolutely possible to do if you play it right, and you get locked out of queues for a while. If it keeps happening, you get locked out longer and longer. And if it becomes a regular habit and you get reported repeatedly for it...then you get a punishment up to a permaban.
It works just like any other punishment, it's just more difficult to determine context so there's a little more leniency.