Is there any android smartphone that is already rooted by default?
No, There is no phone with root access pre-built but there are phones like oneplus with cyanogen OS. 

I hope these links may help you reguarding this topic.

http://www.makeuseof.com/tag/why-doesnt-android-come-rooted/
http://www.androidauthority.com/sell-android-devices-already-flashe-119002/