Maybe you spend 1000$ more for a PC of comparable performance, well tomorrow you need more power, change or add another GPU, add more RAM, add another SSD. A workstation you can keep upgrade it for years, adding a small cost for an upgrade in performance.
An Apple machine is basically throw away: no component inside can be upgraded, you need more RAM? Throw it away and buy a new one. You want a new GPU technology? You have to change the whole thing. And if something inside breaks? You of course throw away the whole computer since everything is soldered on the mainboard.
There is then the software issue, with Apple devices you are forced to use macOS that kind of sucks, especially for a server usage. True nowadays you can install Linux on it, but the GPU it's not that well supported, thus you loose all the benefits. You have to stuck with an OS that sucks, while in the PC market you have plenty of OS choices, Windows, a million of Linux distributions, etc. If I need a workstation to train LLM why do I care about a OS with a GUI? It's only a waste of resources, I just need a thing that runs Linux and I can SSH into it. Also I don't get the benefit of using containers, Docker, etc.
Mac suck even hardware side form a server point of view, for example it's not possible to rack mount them, it's not possible to have redundant PSU, key don't offer remote KVM capability, etc.