Why can’t you turn off your iPhone with Siri?

With the help of Siri (voice interface), people can have a call, add tasks, have a search. I give a command for turning off my phone but Siri said that "It is beyond my skills". I tried again and Siri responded as "I'd like to but I cannot".

Why not?

Why are some of the voice commands like "shutting down the device" not accepted? Is there any user experience related issue with it?

I first thought that it is related to security related issue but this can be solved by implementing same strategies like confirmation/alert boxes? Do you have any solid answer for why Siri can't turn off my phone?

Can the other voice command assistances (ex: Alexa or Cortana) handle such requests or not?