Flex is easy when the players are not too lazy and too stupid.
Normal is easy when the players are not too lazy and too stupid.
Heroic is easy when the players are not too lazy and too stupid.
The definition of "lazy" and "stupid" is arbitrary. Stopping DPS to move out of fire might be fine for most in Flex/LFR but you'll probably catch hell for it in Heroic.
Because it's impossible that the tank is overgeared or geared for more DPS.
The game doesn't teach them how to play and currently it's not required for LFR (and it probably never will be).
Or they ignore you because you aren't a raid leader, no-one is. All players equal in LFR. Why should they listen you?
In WoW, there's always more that you can do. Especially with TI Burdened gear. Hence, players that slack in this regard make the content more difficult for themselves.
In LFR some of the mechanics are tuned such that they can be ignored, so why shouldn't they ignore them?