In my opinion, the reason people keep asking about Pi clusters is
The internet is full of pictures.
They are cheap.
In my opinion, it's because people on the Internet keep calling them "Supercomputers" (which they are not, in terms of performance). That nonsense should stop, but it's the Internet, so... ¯\_(ツ)_/¯
The internet itself could be considered as one of the largest distributed compute clusters in the world.
That doesn't necessarily make it the most efficient one.
Whether you can call it a supercomputer is a whole different topic, because it's tricky to make an Apples-with-Apples comparison with the average computer - what is the average computer, anyway?
You would probably only build a Pi cluster if you were interested in learning about datacenters
or were actually using them as a cheaper alternative for serving a community with automation.