Let me explain this in a very very simple way: buttons which are not literally mechanical switches that physically connect and disconnect from power require that at least some of the circuitry to be alive because they’re capacitive contacts, a technique which requires some power and some logic to detect that the button has been pushed.
So even shitty shit $0.12 microcontrollers often come with support this stuff, so that they can generate a hardware interrupt in the microcontroller to wake it up when a user presses one such soft button to power on a device.
Beyond this, in order to support something as simple as wakeup from the network side - for example, to support Find My Phone functionality - even $3 microcontrollers (not microprocessors, microcontrollers, their cheap cousins with puny computing power) have features such as programmable secondary low power cores that consume tiny amounts of power.
Even this “advanced” stuff doesn’t add cents to BOMs, it only adds tiny amount of extra surface on vastly more complex microchips, which translates to at most tenths of a cent of extra cost because this stuff isn’t supposed to be decoding videos or running some social media user interface (or any user interface), it’s just running small simple programs which might use a few peripherals configured to remain active in low power mode (and those can be network related) to listen for certain conditions and decide if it should wake the main cores up or not.
The functionality isn’t there in the hardware because they added it to facilitate spying, it’s there because that’s just the direction the technology evolved in the last 2 decades - soft buttons instead of mechanical ones, some amount of always on functionality for fast start, support for convenience features for users, that require some kind of wake up from the network side or merelly because microprocessor or SoC makers add everything and the kitchen sink to their designs to try an make that chip usefull for the broadest list of use cased possible (it’s quite insane the amount of stuff built-in in even the cheaper of the the current generation of SoCs) so that those chips are used in more devices and get sold more.
But it gets better: none of this is necessary:
Hacked phones just simulate shutdown. They don’t even go into low power mode, they just show the user a fake shutdown animation and keep on running at full power.
Now, maybe somebody who has never been involved in Politics, or Demonstrations, or Strikes can go around with total confidence that their phone ins’t hacked, but if you’re anywhere close to the organisers of the kind of public demonstration that can snowball into to the current POTUS losing an election, don’t assume your phone hasn’t been hacked (which can be done remotelly) and that turning it off in the soft button marked power when you go into a meeting with other organisers has actually in fact fully turned it off in a way that makes sure it isn’t spying on that meeting.