Working with teenagers

I love my job - genuinely I do. Teaching is great, particularly when you know you've reached someone and helped them to achieve.

But teaching brings with it some unchanging and undeniably frustrating day to day issues - usually circular conversations involving the words "But", "Why?" "I was just..." and that usually end up with "Because I said so!"

A typical conversation goes something like this.....
Me: "Fred, put your phone away, you know the rules"

Fred: "But I was just trying to ring my mum"

Me: "if you need to speak to your mum, you need to wait until break and then go to the office. The rule is no phones in school. You know I'm supposed to confiscate it if I see it"

Fred: "Why?" (said with real whiny voice)

Me: Because it is the school rule Fred. I'm giving you a chance to put the phone away and we'll leave it at that.

Fred: Yeah, but why can't we bring phones to school? Thats stupid.

Me: The no phones rule was brought in Fred because pupils kept using them during lessons and texting. I don't bring my phone to the classroom because it would distract me from my job. You don't need your phone in lesson time.

Fred: Yeah, but, what if my mum wants to ring me?

Me: If your mum really needs to contact you she will ring school Fred, then the office will get the message to you.

Fred: But what if its an emergency. Nah, I'm not giving you my phone.

Me: I wasn't asking you too, but NOW I am.

Fred: WHY?

Me: Because you are arguing with me!
Fred: But I was just trying to show you that the rule is stupid.......

on so we go on....until I finally pull rank and tell Fred...."Because I say so!!!!!"

So my question is
WHY do kids not understand that its easier if they just don't argue!?!?!?!