There's already a power-button. Hold it down, and it reboots.
No, it doesn't.
Press the power button briefly, the screen goes dark. The system has gone to "sleep" to save power and prevent things like pocket dialing. The system keeps running, it has not rebooted. Press either the power or home buttons, and the system wakes up again.
Hold the power button for a longer period, Slide to Power Off appears, you slide, and it's off. Really off. If you then turn it on again, it goes through a full boot-up. Apple considers this to be a "restart."
If the system is frozen/non-responsive, you can hold the power button until the cows come home, nothing's going to happen. Similarly, if Slide to Power Off appears and you keep holding the power button... nothing further happens.
Now... A "reset" is used to force the CPU to restart when the system is frozen/non-responsive. The CPU is given a formal "reset" signal. To do that in iOS, you hold both the power button and the home button until the Apple logo appears. Let go of the buttons, the boot-up continues.
https://support.apple.com/en-us/HT201412
If you were talking about a Mac, it's a different story. If the system locks up, you can hold the power button until it restarts. But people don't carry their Macs around in their pockets, and when the lid of a laptop is closed, the power button and keyboard are protected from accidental operation.
iOS has several fail-safes to prevent unintended circumstances. Slide to Power Off and the two-button reset exist because when you're holding an iPhone (or have it jammed in a pocket) it's just too easy to accidentally press a single button, or press it for too long.
Why should it be hard to do a full power-off? You can't receive phone calls with it fully powered off. People can get pretty unhappy pretty quickly if the phone is off when they expect it to be on.
Why should it be hard to do a full reset? If the system is running, a reset interrupts whatever tasks and processes are running. While iPhones do a pretty good job of constantly saving the current state so that it can resume as if nothing happened... It's not a sure thing. I have a number of games that lose the current score after a reset. Argh!
But the net effect of all this is that Apple continues to depend on two mechanical switches in iOS devices for a reset (the ringer/mute switch is also mechanical - the best explanation I have is it allows us to check the position of that switch without looking at it). As long as there's battery power, a mechanical button works. Components that depend on "higher levels of system consciousness" (like the touch screen) are useless when the system has crashed. Since Apple wants to have a safety interlock on the reset function, both buttons have to function when higher-level functions are not possible. Hence, the two-mechanical-button solution.