People rebuild their own because the stockies are production line items which were made that way to speed up the manufacturing process and cost less meaning they will sell at a competitive price. They Nissan, unlike motorsport teams were not chasing Eleventy-hp out of the engine like motorsport teams want, different end users. If you want a engine capable of that have a look at the likes of the Z Tune or some of the cars built by Nismo, Midori or Omori. Maybe wrong but I havent heard a lot of them having engine failures.
IMO you cant really beat the factory seal of a Nissan built engine like said above, i can imagine if they built these engines with all the stuff people use for their rebuilts those engines would be tough as nails.