I think it's mostly just life around the edges of tiers that's abysmal. I was in my plat promos yesterday morning, and spent the day doing really well individually and losing all my games because people got all salty over silly reasons and DC'd (e.g. one person got mad that the jg leeched xp while waiting to gank) or just straight up failed to connect. (I think theres like 5+ games where I literally have a perfect kda and 50%+ kill participation that are losses >.<)
The plat 5's (in games with golds) are all salty that they can't climb further and/or don't care about winning anymore since getting demoted a tier is hard. Meanwhile in the middle of a tier everyone is generally still trying to climb and all working together for their own best interests, so they'll at least pretend to be ok with atypical picks in the interest of team morale (I supported an irelia adc earlier today - we won). It's not just the gold/plat edge either, I think the edge of bronze/silver and silver/gold is a mess too, and I hear plat/diamond is pretty rough around the edges too. (I'd like to think diamond/master people are pretty serious at that point, but that might be naive on my part.)