Why can't you set full length songs as ringtones on an iPhone?

I've never owned an iPhone but i heard you can't set full songs as tingtones, i'm just wondering why? The phone is so advanced, why a full track can't be set as a ringtone! Maybe some of you understand Apple's reasoning on this.

Maybe because a ringtone is made for you to answer your phone, not listen to a song?

If you were calling someone and wanted/needed to leave a message if they didn't answer would you like to hear the phone ring for the duration of a song just to get to the message mashine? I wouldnt want to hear my phone ring for 3 min.

The person who is calling you won't be there for a FULL song waiting for you to pick up and if you wanted to listen to a full song you can use the iPod feature. It's just easier that way to use.