Like most of these “oh my God so brutal” execution methods, scaphism probably never actually happened or is at least exaggerated. Ctesias wrote the only first-hand account of the punishment ever being delivered and his credibility among modern historians is a bit… questionable. Plutarch’s own writings on scaphism were lifted straight from Ctesias, as well.
Even if scaphism did actually happen like Ctesias wrote, it’s not like it was a normal thing.