At higher power the Ethernet drops quite a bit of voltage/power compared to the wiring in your walls. Furthermore 48v vs 110v is double the amperage/more loss on top of that.

Then there’s double/sometimes triple conversion (120:48 and then 48:dc; 120:48 and then 48:12, and then 12:dc).

Furthermore magnetics are a must on both side of the PoE which also isn’t great.

At lower power there’s more circuitry to run and multiple conversions aren’t great compared to a simple cheap flyback.

For more technical feel free to check here, although it isn’t quite end to end: https://e2e.ti.com/cfs-file/__key/communityserver-discussion...