C, is high level language that happens to map really well to hardware. Much of UB is UB simply because it difficult to effectively detect when something goes in to a state of UB. Writing to a memory address can be done in one instruction, but figuring out if that address is legal to write to is a slow and complicated process. The C standard let an implementation ignore that problem, but it is also does not stopping the implementation from making sure the address is valid before writing to it. Almost no one tries to make such an implementation, because the users have decided they would rather have speed, but anyone can write a ISO compliant implementation without any UB if that's what they want.