Because a meaningful guesstimate for tuners needs a workable sample size, and it would generally be too close to zero to tell if any exist. At the time Fermi l

Because a meaningful guesstimate for tuners needs a workable sample size, and it would generally be too close to zero to tell if any exist. At the time Fermi lived a metric shitton of abodes housed a piano-- that's almost certainly why he used that example. You could just peruse a block of abodes to get a decent estimate (or simply think of one in your head). But in general there are fewer tuners than there are pianos, so even at the time a rough estimate for a city block could return zero. Even today, if I walk a city block I'll find six buildings that house a piano. But I'd probably need a 20-mile radius before one of two piano tuners come into range. Another question occurs to me-- when I peruse the pianos in the block, what is the chance I'll actually see one of the tuners doing the tuning? Smart tuners leave their card inside the piano with a list of dates of tuning-- if I have a chance I'll sneak into one of the houses of worship and see what the frequency is. I'll bet it's more likely I'll spot some deer meandering on the way.