Why doesn’t the power at 4 ohms double compared to 8 ohms?

Summary: If an amplifier “doubles” its power into 4 Ohms, it often means the 8-Ohm rating was “under-built” relative to the power supply’s potential. If an amplifier provides the same power into both, it means the power supply is being utilized to its maximum optimized potential at the higher impedance.
Refer to this guide: https://nuprimeaudio.com/power-impedance/