But that’s not what’s happening here - the model here is that every piece has a uniform probability of being selected next, in which case every possible subsequence appears in the limit.
While a sequence where one possible subsequence never appears has probability zero in the limit, it’s still a possible random outcome. Incidentally, every concrete infinite sequence has probability zero.
E.g. it might be that there will never be "I" piece at all, even for infinite random sequence. Yes, probability of that happening is exactly zero, but that might happen.
For example, if we select at random any number between 0.0 and 1.0 -- probability of selecting it is exactly zero. But we still selected it.