because it's not the power button. i was in possession of an iPhone XS for a brief period of time, and I was baffled to discover that holding down the power button did not bring up the "slide to power off" dialogue one would expect. instead, for some ungodly reason, it summoned siri.
it's hilarious and absurd that there is a significantly measurable number of people googling how to turn off their iPhones, because Apple saw fit to inform nobody that they changed it so you now have to hold the "side button" plus either volume button. It's completely unintuitive, why would anyone think that the action to turn off their phone would be "take a screenshot, but longer."
I looked up how to change it back into behaving like a standard power button because I vaguely recalled reading that that was a thing... I got results all concerning Samsung Galaxy phones. So, as usual, Samsung took the bad from Apple, but at least gives you the option to turn it off. Presumably, on iPhone, this is just How It Is.