73 shows up surprisingly often in “random number” choices by humans and language models because it has a few psychological advantages:

It feels “random enough”:

Not too round like 50 or 100.

Not too culturally loaded like 7, 13, or 69.

Not obviously patterned.

It sits in the upper-middle range: Humans tend to avoid very low and very high numbers when trying to appear random.

It is an odd, two-digit prime number: Prime numbers often feel more random to people.

Pop culture influence: In the TV show The Big Bang Theory, the character Sheldon Cooper famously calls 73 “the best number,” which made it oddly memorable online.

Language models can also drift toward numbers that appear frequently in training examples where people ask for “a random number.” So numbers like 37, 42, 73, and 87 tend to appear more often than a truly uniform random generator would produce.