A significant number of implementers roll their own UUIDv4. It seems so easy so why not? Most UUIDs are used in contexts where the devs are not that sophisticated so it isn’t that surprising that naive mistakes happen. If you are using it for distributed UUID generation, it just takes one person making a mistake to create havoc.

UUIDv4 is banned in many high security environments primarily because it is easy for people to screw up in practice and it is difficult to detect when those mistakes are made. 128-bits doesn’t leave much room for mistakes using probabilistic uniqueness.