I've never seen a guitar amp with the power switch on the back, at least it isn't not the norm. But why bass amp has them on the back?
Fender Twin has. The most normal amp there is. Never seen one of those? And it comes from the old days of tube, when you had a standby switch as well as a on/off switch. To let the tubes warm up first. It had to do with a lot of things. But most tube amps has switch at the back AFAIK. Space constraints is one thing, that they couldn't be fit at the front. I e they wanted controls and switches that you just reach for as much, as the tone and volume controls to be set out front, visible. There's just the on/off LED that is visible that you need to check to make sure it's on or off. You reach back ONCE, and then you leave that switch on for good. No need to tinker with that. And it's hidden from inadvertently knocking it on/off. Sort of slightly more protected than when in front.
Another thing, although a tad far fetched, is that they really wanted to impose a space at the back, so any amp (at least combo) wouldn't be too close to a back wall in order to have some air. If you can put your hand in there without problems, chances are that the ventilation from the tubes and heat dissipates upward. If you had everything at the front, chances or risks are that you push the cab flush with the back wall, which can make havoc if you have and open combo cab or not. In sound too.
I do agree, that it is always hard to guess which switch you turn (is it the standby or on/off?) when reaching back there without peeping, but you learn pretty quick which on is the fartherst right or left and so on. What I don't like though, is if that the actual led or lamp for showing if it's on or off is placed too at the back. That one beats me.
You need to get more out and around. Get around. There's numerous out there that has both. Top loaded combos, front loaded, so I don't really get what you got that notion from..."never seen any" must equal "there isn't any". Funny selective deduction.