I'd guess it's because they have a high quality switching power supply, a power factor corrector, and a digital amplifier. All those things require good capacitors -- probably more than if all the circuitry was linear.

500W is a lot of juice. Consider that 120 times per second, the amp gets no power from the wall and has to rely solely on the power in capacitors. The capacitors have to supply the full 500W. Possibly more—something that’s nominally 500W “peak” but driving a speaker may see instantaneous loads much higher.

Modern circuitry has increased efficiency and reduced the heat dissipated by an amp, but the capacitors still need to supply the full amount of power between mains cycles.

Perhaps it's a measure against failure? Do these capacitors fail open?

Or price, although that would raise new questions. Another is availability of components. Or perhaps the form factor (these capacitors seem slightly less tall than the ones on the other board in the picture).

Designing for failure seems unlikely here. I think they just wanted lots of capacitance and a low ESR. Lots of capacitors is a cheap way to get that.