Why is leaving such an extreme offense?

As title says. I never understood why leaving is so against the rules to such an extreme. I can understand abusing it but its like, i've had a few games where people would argue at champ select and end up picking their champs anyways when where they want to go is taken. Like I had games where we would have 3 supports and no adc or mid since people are dense and all wanted bot. Or games where its clear you lost, you have a score like 10 kills vs 40 and they have all but your base turrets or even those too while they have all their turrets still and every dragon and yet you get those 2 guys that keep saying no to surrender when you have 0 chance of coming back from that. Like I had a game with such scores and even the support like blitz was owning us it was that bad.

I just don't get why its so horrible to just get out when you see such senarios. I am not one that leaves games but when I see chaos in champ select and we aren't even in the game yet, I want no part in it. Or if we are in a game and its already clearly a loss but you have some people that refuse to surrender that I don't see the point of staying. I can understand if you have a chance to comeback but let's be real, those aren't all the time, most games are usually you get facerolled or the enemy does. Its literally pointless to continue such games, all due to 2 hard headed people deciding they are either trolling or to be blunt, stupid and think we still got a chance.

I am fairly new to Lol so why is it such a big deal? Like I know having a leave in the beginning or during an actual game is extremely bad, but when the game is so unbalanced that you got no chance, why is it so bad? Or when the game hasn't even started and you got people cussing and arguing, why it is so negative to just dodge the match and save yourself 20-60 minutes of hell.