Theres probably a cost/ durability element. You want that thick metal plate to be stainless steel (expensive), or protected in some way (slightly less expensive

Theres probably a cost/ durability element. You want that thick metal plate to be stainless steel (expensive), or protected in some way (slightly less expensive), wood ash is caustic, so could eat though unprotected steel quite quickly if not properly maintained. It may be possible an over enthusiastic bbqer to overheat the metal plate, I don't know how likely that would be in real life though.