Because while it might be annoying, it is not a threat to the amp, so they don't go to the extra expense. The pop is limited to the power rail voltage, just like the signal is. SO the pop will never be any louder than the peak signal can be.
Isolate the problem. If you want to see if it is the power amp settling, plug something into the power amp in jack to disconnect the preamp signal, then cycle the power. If it no longer pops, the preamp was the popper. If it still does, then it is power amp.
Likewise, you can send the preamp out signal to some other amp to monitor it and see if the pop comes out of the preamp.
SOme amps pop because the power switch arcs as it opens. The power supply charge lasts long enough to amplify the resulting spike. To test for this, leave the power switch on and just pull the power cord from the wall outlet instead. If it doesn't pop that way, then the power switch is sparking.
In my experience, when I see any of the Fender solid state amps with that wedge shaped chassis - front panel short, rear panel tall with sloping chassis bottom - come into my shop, I expect to have to resolder the two main filter caps sticking up from the main board. They never seem to be bad, but they love to crack free of their solder. If yuo still have the amp, do your customer a favor - glob a bunch of silicone sealer or hot melt glue between those caps to glue them together, They'll vibrate kess. Then run a bead of glue around the base of each.