A online UPS has in line: a rectifier with its transformer and output-smoothing capacitors, a huuuge battery, surge capacitors, power transistors, another transformer and more capacitors. And only then comes the actual device with its PSU.

It takes an awful lot of power to smash through these multiple layers of insulation and bypass all the capacitors, at least if the PSU is properly designed (specifically, clearance between the various power / ground domains).

In contrast, Ethernet transformers are tiny small little things.