Imagine if you were generating 16 UUIDs per nanosecond, every nanosecond.
According to [1] due to the birthday paradox, the probability of a collision in any given nanosecond would be 3E−17 which of course sounds pretty low
But there are 3.154e+16 nanoseconds in a year - and if you get out your high-precision calculator, it'll tell you there's a 61.41% chance of a collision in a year.
Of course you might very well say "Who needs 16 UUIDs per nanosecond anyway?"