Compared to other SBCs, Raspberry Pis have been pretty inefficient for a while. A Pi 5 idles at about 3 W, which is pretty terrible for such a board, compared to other similar devices. You can get X86 PCs that idle at 3 W which are way more powerful. Other ARM SBCs use less than half that at idle and similarly less under load.

There are probably multiple reasons for that. The Pi’s SoCs have always used rather old process nodes, which are more power hungry than more modern ones used by other single board computers and PCs - 16 nm for the Pi 5 SoC and 28 nm for the Pi 4. Also, with the Pi 5 there is this additional “south bridge” chip which is attached via PCIe. This consumes additional power and for some reason the PCIe link is configured such that it never enters power saving states. I don’t know why.

Also, the power supply circuitry on the Pi 5 is far from ideal with its 5 V / 5 A power supply. Such a low voltage at such a high current can easily cause additional losses on the wire. That’s mostly relevant under high load though.