On Thu, 15 Feb 2018 10:48:48 -0500, Keyser Soze
wrote:

Society is similar among many Western-style countries...and yet only one
country has the numbers of and sorts of mass shootings under discussion
here.

Which country is that, and what is different about it?

First "society" is not really similar at all and I suppose the answer
you are looking for is we have no real connection between mental
health services and any other regulatory agency.
A person can be a ticking time bomb, walking among us, well armed and
the doctors are prohibited by law to warn anyone.
Part of that is, nobody trusts doctors to give unbiased opinions. The
other part is, people with perceived mental problems will be severely
discriminated against and not just by gun dealers.