Why does iPhone not have the option to require a password to turn the phone off?

When someone steals your iPhone, the first thing they do is turn it off, not allowing you to use the GPS function to search for the phone. This means you have 0 time to get a rough idea of where your phone is, as this may still be very useful in tracking your phone.

Is it because:

1. They not thought of the idea
2. Apple wants stolen phones in order to increase sales
3. It's a stupid and pointless idea to begin with

Number 3. When someone steal your phone, it with the intent of eventually turning it on. If you have "where's my iPhone" activated, you can identify the phone as stolen, so when they do turn it on, it won't let them access it, and will notify you.
The idea of requiring a password to turn it off is pointless; the battery will eventually run out anyway, and it will effectively be turned off.

I don't see why someone would steal a phone & then turn it off. Yes doing that would eliminate the possibility of being tracked but then making the point of stealing it, null if you aren't going to do anything with it.

In any case, Apple has measures in place to make stealing an iPhone pointless.

Find my iPhone which can track, erase, lock, sound the iPhone
iCloud Activation Lock which would require the owner's Apple ID & password to be able to do anything as far as a restore/reset

Requiring a password to shut down is more of an inconvenience than anything, particularly to the owner. Apple wanting stolen phones makes no sense since they have no control of what happens to the iPhone once it is sold to a customer.