I use sets a lot, sample size of one. I do data science/engineering stuff, sets of columns/sets of keys. I'm very sceptical of your claim that most developers don't use sets.
Size sample: a few hundred students and colleagues over 15 years.
It make sense: for columns, it's quite commonto use dicts, or pandas dataframe. The sets of key is just the dict keys memory view.
A web dev rarely needs sets. A GUI coders, a sysadmin or a geograph neither.
It's not that sets are not useful, it's just that in the huge numbers of things you need to do in programming, accross all fields that Python attends to, they are pretty niche.