The point of these sorts of questions is not to find people who know how many piano tuners there are in the world, or even to arrive at a reasonable estimate (without good data you're going to be off by miles).
The point is instead to find people who have developed the ability and habit of "back of the envelope" calculation. Not everyone has developed this skill but it can very much elevate the level of many technical discussions. Simply being able to come up with a guess that is within a factor of 2, 10, or even 1,000 (depending on context) can be critically important and impact decisions. Such crude calculations can also serve as the skeleton from which to create more robust calculations with better data. Enrico Fermi was able to estimate the yield of the first ever nuclear bomb test using only his eyes and a few pieces of torn paper blown by the blast, he came within a factor of 2. Sometimes you don't have the most precise measuring instruments or complete data, being able to cope with that and yet come up with useful results is also important.
The point is instead to find people who have developed the ability and habit of "back of the envelope" calculation. Not everyone has developed this skill but it can very much elevate the level of many technical discussions. Simply being able to come up with a guess that is within a factor of 2, 10, or even 1,000 (depending on context) can be critically important and impact decisions. Such crude calculations can also serve as the skeleton from which to create more robust calculations with better data. Enrico Fermi was able to estimate the yield of the first ever nuclear bomb test using only his eyes and a few pieces of torn paper blown by the blast, he came within a factor of 2. Sometimes you don't have the most precise measuring instruments or complete data, being able to cope with that and yet come up with useful results is also important.