Related question: why are concerts now amplified to a deafening volume, to the point that they're not even enjoyable? What's with that?
Because if you can't use the speakers as a defibrillator, something something can't hear you, sorry...
I have no idea. I'm OK with it at concerts, but it does annoy me greatly when churches hit the painful point.
I have a theory about this, and it's directly related to inexpensive high quality speakers and amplifiers. In the 1930s, the only time that you had loud pumping music was when a big band was blasting it out. In the 1960s portable transistor radios started to be a big deal but they all had tiny tinny speakers . . . tube amplification was delicate, generated a lot of heat, and was still usually attached to pretty low efficiency speakers. By the end of the 80s/early 90s loud stuff started getting really cheap. This trend has just continued and intensified through the 2000s. . . to the point that for a couple hundred bucks anyone can throw some giant woofers in their car and throb away at other people to their heart's content.
Because it's so cheap to be loud, more and more people are getting used to loud as the new normal. A concert can't be concert volume any more . . . it has to be deafening because otherwise you could get the same experience at home with your 400$ stereo and an album. Throw in the modern dickishness of cell phone users, and theaters/musicians have even more reason to try and drown out all the electronic devices.
There is going to be a very large portion of my generation in need of hearing aids in their forties and fifties.