5 to win 1 to lose

It is all it takes in league. Just a single bad teammate can and will lose you the game. Winning 4v5 is next to impossible. AFKs = guaranteed loss unless the opponents are bad and really screw up. Matchmaking places emphasis on duos and up so if you go in solo you already start with a disadvantage. With the slight exception of top lane, any other lane that feeds, has an afk, or just plays badly will most likely cost you the game. Top lane is at least semi utility based so if they are not ahead they still can be useful in most cases by being tanks and/or supply hard cc. minion mechanics make 4v5 harder to win since pushing back waves becomes a chore and the supposed safer far is nonexistent. With so many options for anti vision (pinks oracles and sweepers), maintaining some vision control is next to impossible as well.

I remember when RIOT implemented some mechanics to help teams with an afk not be completely shut down like the increased experience gain. But now it is "Aw to bad for you. Should have had 4 other friends to make sure that didn't happen." And with new dragon buffs being all for tower seging, not having the manpower to contest really sucks when you just have to watch them get free buffs that help them to take already easy and uncontestable towers.