Because I usually am no.1 on dps and healing and my co-tank is in dps spec/gear or have no clue what to do in the fight. Also people usually look to the tank for advise when they need to improve on a boss and I cba telling every1 what to do when they could just press the dungeon journal and read everything they need in less than 3min.
People always blame the tank when something goes wrong.
This only goes for LFR as you mentioned in your opening post. I love to tank in raids with my guild and the occasional pug. People are just nicer there.