The problem is that good live sound is highly dynamic. If you ran the PA at exactly 85dBa all night, it would sound very flat and unengaging. You want to push the volume during lively upbeat songs and bring it back down for an intimate ballad, you want a bit of a lift during the chorus, you want natural crescendos and diminuendos. Really good engineers can maintain this sense of dynamism while keeping the overall levels under control by using time-averaged metering, but systems capable of that are quite costly.
Achieving a consistent volume level throughout the venue is much easier said than done. Normal loudspeakers follow the inverse square law, so the sound will be much louder close to the PA system. You can't just put a bunch of speakers all over the room, because you'll get weird phase effects - sound moves at about one foot per millisecond, so additional speakers tend to cause very unpleasant delay and comb filtering effects [2]. This can be mitigated by the use of electronic delay compensation for the additional relay speakers, but not eliminated entirely.
The inverse square law can be evaded by using a line array loudspeaker system [3]. Theses systems use multiple loudspeakers in a phased array, which produces a cylindrical rather than spherical wavefront. Line arrays give a much more even distribution of sound and also reduce the amount of energy that is reflected from the ceiling and floor, providing a much clearer sound with more accurate transient response. Historically, line arrays have been expensive and difficult to configure, so they have been confined to large venues and high-end touring systems. Compact integrated line array systems are now available from many manufacturers, so hopefully we should see an improvement in the general standard of live sound over the coming years.
[1] https://noiselimiters.co.uk/scx-noise-limiter-and-contactor-...
I was thinking more having an RMS max target dBA/dB(SPL) and a cut-off above that to limit instantaneous volume rather than requiring a particular level for the night.
I'm in the UK; I know a little about phase effects having set up a few room level PAs (5 instruments, 3 mics, 6 speakers, single board; that sort of size). However, I'm not familiar with large set-ups as I've only attended smaller concerts of the order of 100s-1000s of people.
It seems very like colour profiling to me. Graphic designers seem to match to objective standards though, whilst sound techs seem instead to use their own perception at the time?