Wood is porous & absorbs all kinds of bacteria, mold, etc. Basically you're exposing the next occupant to anything that the previous one had, plus it allows new pathogens to build up. It's really not sanitary unless well-coated with something non-toxic so it can no longer absorb everything, including fecal material.
Also, plywood off-gases toxic fumes, which are far more dangerous to a small animal that's breathing them all the time. Plywood is made in layers that are bonded together for strength- it's very useful for furniture & buildings- which at least are open to the air- but it's best to minimize exposure whenever you can- it's the adhesives/glues bonding the layers that are the chemical issue. Used to be that plywood off-gassed a fair amount of formaldehyde but they're finally cracking down on that IF it was made in the U.S. after 2018, but that's not saying there aren't OTHER things used that may be harmful to small creatures kept inside. Also, it apparently still off-gasses SOME formaldehyde, because the law only says it can't be excessive (ie. over a certain amount) and believe me, they didn't test it for snake safety.They're only worried about human safety, & we're much bigger & better able to withstand such things- plus, we're not living in boxes made of uncoated plywood with minimal ventilation. The regulations relate to safety for how it's typically used.
Plywood splinters are another concern. I've gotten my sharebut your snake can't complain to you, much less remove them or treat their own wounds.