If one considers it for a moment, this is obvious from a sound engineering standpoint.
Front of house requires the engineer to have individual level controls over the mix. If the guitar player gets 12 heads and 24 4x12 cabinets, there's no way other players - bass, drums, vocals, synth, etc - are ever going to be able to compete. Even if the front of house system could compensate, the mix is always chasing the guitar volume - giving the guitarist control over the FoH mix when they can't even hear it.
Typically the guitarists amp will be mic'd or run as a line out signal to the front of house board. What the audience hears is what's coming out of the house mains not the players' individual amplification systems. 4x12 and 8x12 guitar rigs are still relevant because it provides a certain kind of sound, but in large live venues, the guitarist's rig is not about volume.
But the wall of amps is merely for appearance - especially in today's era of high-tech live sound. Originally you would have just left the amps unplugged, but given the modular nature of guitar amp construction, its easy to see the value engineering: remove the speakers, then remove the amp electronics, and eventually just resolve to building only as much cabinet as is necessary to provide the appearance.