First off… pros don't just decide they want to turn their amp up halfway through a gig. That's what sound crews are for; there's one guy whose job it is to make sure everyone on stage can hear themselves clearly.
That aside, at a large gig it wouldn't make the slightest difference front of house. There's not really a great deal of the actual amp's volume that makes it past the third row.
At a small gig, the front of house engineer will endeavour to balance what is coming from the stage with what is coming from the front of house reinforcement.
At this type of gig you may not have a monitor mixer to help you out with onstage sound.
The 'cool' way to deal with this, if you don't feel you're loud enough [even though you said you were at sound-check… that's what sound-check is for], is to trust that the FoH engineer got it right & the audience can properly hear what's going on.
There's also the practicality that the band is very used to what this traditional, all-in-a-row setup feels like on stage. Everybody is nearest their own amp, which leads to a natural balance from the stage, as every player can hear themselves 'best'. They can correct for the room without external guidance, just because they've done it so often. They're used to playing without FoH help & they're obviously pretty good at it, because they're working a lot. If they weren't any good at it…
At a tiny gig, sometimes you just have to put the gear where it's practical. A well-seasoned small band can balance this up even without a FoH engineer within about 10 minutes.
If you're not truly seasoned & used to this, then if you turned the amps to face in random directions, or worse, at the stage back wall, then you would be projecting a bass-heavy but otherwise quite unknown quantity around the stage & pushing into the main venue. At anything bigger than a pub gig, you'd just be making FoH issues for yourself. Without a monitor mixer, you've just made your life hell - on-stage you're getting freakish standing waves at the bottom, 100Hz or lower; no-one can hear themselves properly [& possibly more importantly, no-one else properly because your known-lineup amp distances are changed]… because you don't have good on-stage monitoring. The FoH engineer has to try cut all that weirdness from the FoH sound. an amp pointing stage left is going to be contributing far more to what the audience can hear at the opposite side of the venue, whilst those nearest it will only hear the bottom end. Most FoH mixes are essentially mono to try avoid this.
In short… they don't do it that way because it doesn't even work in theory. In practise I bet there's more than one band who have tried it - yet no-one actually kept it as a working practise.
I'm not saying it can't ever be done, but it's probably not worth the hassle in most venues if you can fit in a regular line-up..
