For the same reason that "studio monitors are 'flat'", that "mo' louda is mo' betta", "85dBSPL is the ideal monitoring level when mixing", "mastering is all about fixing the mix" and all those other widely-believed untruths. Because that's what The Great Unwashed paste all over the Internet.
When you are trying to showcase a guitar tone, don't bother with the drums and compressors. (Was there any bass? There was no real low end to the mix)
Sure if you want your ears to be ringing constantly by the time your 40.
These unenforced standards are a little more strick than OSHA's but it gives you a good idea. Lots of docs don't think these are strick enough. What you need to be careful of too is the length of time you are exposed. Mix at lower levels and then just turn it up a short while to check how it sounds louder. Ringing ears is a bitch!
It's great to use constantly if you want your bass mix to sound lacking or if you want to build up the potential for possible long-term hearing loss. Nothing wrong with checking mixes at 85dB or doing part of your job at that level, but it's not something that most pro engineers do for very long and there's certainly nothing magic about that volume like the Big Myth leads one to believe.
