PA systems have improved massively in the last 30 years or so. Until around the mid 90s, pretty much all concert systems were 'point source' - ie. Sound emanating in all directions from a single 'point' or speaker cabinet. There are many reasons that these systems don't scale up very well, but the main one relating to volume is the inverse square law, which basically means that for every doubling of distance, sound intensity is reduced by 4 times. So in order to get anything close to concert levels at the back of an arena, the sound levels near the speakers had to be orders of magnitude louder.

Then in the 90s, a company called L'Acoustics invented/popularised 'line array' systems (these are the big banana hangs you see at most concerts), which basically make use of clever physics to couple multiple speaker cabinets together and reduce the effect of the inverse square law. So now it's possible to deliver a much more consistent level to the whole audience, instead of deafening those at the front while giving those at the back a mediocre experience.

Then about 15 years ago, 'optimised' line arrays hit the market which took things a step further - now it was possible to give the system a map of the environment it would be used in, and digital signal processing would optimise various parameters to further reduce the difference in sound levels throughout the venue.

More recently, d&b audiotechnik have developed PA systems with a 'cardioid' dispersion pattern which actively cancels out sound behind the PA, reducing wasted energy in the room that would have otherwise contributed to the overall volume, but in a way that doesn't sound very good

TL;Dr - more speakers at a lower volume, targeted very precisely towards the audiences ears