It's a flawed argument at its core, you're not buying a power supply, you're buying an amplifier with its own power supply, the performance you see includes the performance of the power supply. A power supply integrated into a case means a much larger case and much greater cost for performance that perhaps could be better, but at what cost?
As I said, the strength of these products is the quality/price ratio rather than the performance in absolute terms, from this perspective the choice of external power supply is by far the most sensible one.