Cynically: you should not want to switch off your device, at least not often, so this function is deprioritized. If you want it off for reasons other than long-term storage, something in the design is lacking.
…which I do countless times a day, and apple put their “screenshot” combination buttons on opposite sides of the screen, EXACTLY where I hold it.
Because if I don’t - it butt-dials random numbers. And by random - I don’t mean digits, but something a lot worse - a russian roulette of employers, acquaintances, municipal services, businesses or anything else that happens to be in recent calls, or contacts, or numbers that dialed me, or that I dialed, or… you get the idea.
Also many venues like cinemas, theaters or planetariums require you to turn the phone off for the duration of the performance.
Also you may need to save battery, e.g. on a long trip with no way to charge the phone.
To save battery on a long trip, I combine airplane mode with low-power mode. Then my iPhone 8's battery level does not move.