Same reason i didnt understand why people use stainless screws and marine grade plywood. I did use stainless screws for the plywood just to keep moving forward though wasnt worth gettin into itI don't know the answer to that, but in most builds I have seen they either Bondo over or wood filler over the screws. I always assumed it was a possible weak spot so you would want it as reinforced as possible. I could imagine under all that pressure water some how finding a way past the epoxy could use the screw threads to make an escape.....but if it gets through the epoxy aren't you kind of boned anyway?