Are there any other reasons? Possible safety risks? Certifications needed? Note that my product has 380V AC inside, but the front PCB is galvanically isolated.
In Europe, if your case has conductive parts, it needs to be grounded. So, unless you're only using a single-sided PCB with no metal going through the PCB substrate, that will require a three-pin connector, which is a big no-go in a lot of situations. In the other situations, using a bit of shaped sheet metal as enclosure is probably a lot easier (not necessarily cheaper material-wise, since you'd need a tool made to build the exact shape of metal, but easier to mount things in, easier to handle, store, test). I am not a lawyer or product design expert – but probably there are more reasons than these.
Also, 380V AC sounds a lot like you want a stable enclosure. PCBs aren't meant for this kind of applications – they are relatively brittle / not shock resistant.
In all cases, a letal-voltage enclosure will be required to be "reasonably foolproof". Dropping the thing shouldn't expose the internals. Building that from PCBs sounds like a lot of structural elements (standoffs, guides/shafts/profiles…) need to be added to make a PCB enclosure reliable. That will have inhibitive complexity.
Also, and this will vary very much with legislation, exposed edges of fiber glass material themselves might be prohibited. You don't want to shave of fibers. Again, this can be solved with using non-PCB edges/guides and whatnot, but at that point, the PCB becomes more of a problem than a solution.