I think for the same reasons that 128bit ones can collide, any amount of bits could. I don't have a perfect understanding of the math involved so I could be wrong.
Basically in theory you could generate 1M UUIDs per second and never have a collision for the life of the universe. But in practice due to bad entropy some collisions have been seen in practice. I made the service in part as a joke off of this discussion, but also in case anyone may actually want to use it, it will dutifully provide the advertised function.