For a 4 layer PCB, why are the power and ground planes usually inner layers

mostly because most components today are surface mount so putting pads on the surface of the board makes sense, and then running traces on the same surface as the pad makes sense. Vias (the metal plugs that let traces change layers) are usually wider than traces, and they basically block all the layers they punch through, whcih means that the more vias you have the more board congestion you have.

So, if you've got surface mount components, you'll put the interconnected components on the same side, and keep the traces on the surface, and avoid vias as much as possible.

That will open up the inner layers for nice, big, fat, juicy traces for power. The bigger, the better. If you can make pretty much the entire inner layers power and ground, you get capacitive effect between the planes and that means that as a chip suddenly draws power, the power plane acts as a cap and puts more current into the part, keeping the voltage constant. Without that effect, you might have a power cap placed near every component. More parts, more cost. If you have ot have 4 layers anyway, maybe the board capacitance will let you remove some parts from the design. Maybe not. But couldn't hurt.

When through-hole components were the standard, you *had* to have a via everywhere you had a pin, at which point, the trace connected to a pin could go on any layer, the pin already goes through the entire board, so you just tap off whatever layer you want, and start your trace there and run it to its destination.

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