I would say it's a certification problem. Technically I see no problem why it shouldn't be possible. Lots of airplane take off automatically (because they don't have a pilot in the cockpit).
The reason why it's not certified is most probably because pilots have to manually abort if something goes wrong. An automatic could also decide but that's (fortunately) not yet the case. So if you would have to switch of the autopilot first and then abort the TO this would cause more delay and maybe more overruns/other incidents/accidents.
Just my thoughts, no regulations to back it up.
Dani