The difference is simple, a 3d printer starts with nothing on the 'table' or platform and adds. It has 100% of the information of what is added, so in theory it's possible to know where any potential collisions are (this is not done in practice, and also doesn't account for adhesion print failures, etc, but it's possible).

A CNC starts with an unknown block on the table, held by an unknown workholding fixture, made of an unknown material. There just is not enough information to not crash into something unless programmed around it. Or just run way too fast and destroy tools.

In theory you could design sets of rules, but now you're having to add so much specificity to a design it's a big time waste. There's not really any meaningful entry level CNC machines, so if you can pay for the big machine... you kind of can pay for the expertise to not destroy the machine.

There are FDM processes that go back down in Z direction - like printing multiple units on the same build plate one at a time. And in those situations you do have to be careful not to crash the print head into an already printed part.

Most will print every copy at once, switching between them.

It also often works better because doing small parts can overheat the plastic. Moving between copies lets things cool down.

It can be useful for circumstances where you want to minimize (or even eliminate) retraction. I've done some cool prints in vase/spiralize mode with large nozzle sizes -- with zero seams, zero retractions, and zero moves over printed surfaces, you can get some really awesome looking prints.

But anyway it was just an example of a scenario where the slicer might not have enough information to know how to not crash the print head.

Almost noone does this, though. But, even then, you have all the information you need in theory because you're putting down the plastic.

FDM slicers usually don't have the information about the dimensions of the print head. Yes, it is a rarely used feature that is only supported by some slicers, but I just mention it as an illustration of the similar issue that exists with CNC machining.