What's wrong with a 60% winrate? Last season I had a 78% winrate with Syndra but that's only because I started in Silver 5. If someone's only played a champ after they got to an appropriate place for their skill level, and they're winning about 50% of their games, then 60% winrate is just fine. I have like a 52% winrate with my current main over like 140 games.
Anyway, addressing the actual topic, I think it's partly due to frustration, and partly due to a lack of caring. I blazed my way through gold this season (previously I had never been higher than Gold 5) but as soon as I hit plat, I started losing like EVERY game. My overall winrate dropped from like +20 to -10 (my Azir winrate is now 3-14 I think). At that point I sort of just stopped caring after a long time of a LOT of frustration. I just picked whatever, and it actually helped me a lot. I had a lot of fun picking champs that I never really played, even if I didn't do so well, I figured I didn't really have anything to lose. That actually helped me a lot in getting out of my slump, although every time I get up to Plat 4 pretty much the same thing happens until I get demoted again, which is why my winrate is still like -3.
tl;dr
People stop caring when they lose a lot.
It can actually help sometimes.