It comes in a couple common flavors. Most commonly it is people just rolling their own implementation and using a PRNG or similar. Not every environment has a ready-made UUIDv4 implementation, and not all UUIDv4 implementations in the wild are strict. A rarer horror story I've heard a couple times is discovering that the strong RNG provided by their environment is broken in some way. Both of these cases are particularly problematic because they are difficult to detect operationally until something goes horribly wrong.

The main reason non-probabilistic UUID-like types are used for high-reliability environments is that it is easy to verify the correctness of the operational implementation. It isn't that difficult to deterministically generate globally unique keys in a distributed system unless you have extremely unusual requirements.