Probably because religious apologists use an extreme definition of the word, to dismiss arguments with a type of poisoning of the well argument.
"The OED defines scientism as…
thought or expression regarded as characteristic of scientists."
This is in stark contrast to how religious apologists use it, where I have only ever seen it used as pejorative term.
However even the most extreme version is has a more reasonable evidentiary basis than unevidenced religious claims for magic and the supernatural.
Here:
“…belief that science is the only source of reliable knowledge and that the methods of the natural sciences can solve all problems. It holds that scientific methods, like the scientific method and empiricism, are the only valid way to understand the world and are capable of explaining everything, including subjects typically addressed by philosophy, religion, and the humanities. This is often seen as an extreme form of scientific realism that can dismiss non-scientific fields of knowledge.”
It’s an objective fact that science has surpassed all other methods in it’s success at helping understand objective reality, it is a purely subjective claim that the supernatural exists, so even the most extreme definition of scientism has at least some evidence to support it, though most here would stop short of the claim above, at least in it’s entirety.
I still think philosophy has some use, unlike religion beyond it being a crutch to provide succour through wishful thinking, which often does more harm than good.
FWIW the original use of the accusation by @TheMetrologist, was in response to @Keith77’s post, where he made no such assertions, merely decried the religious claims around him as originated from archaic superstition, and hoped aloud that they would joint modern scientific era. Given that in some countries religions are anti-science and intellectualism wherever they perceive a conflict between their religion’s claims and science, often to the detriment of people who might benefit from say medical science, it is seems a reasonable enough post to me.