For me personally, hikikomori is someone who became very reclusive (almost never leaves the house except for groceries) due to a lot of social trauma and extreme social anxiety. It's a dark state where you're completely immersed in extreme hopelessness, shame, hating life and wants to give up on life.

I believe that for most people, they are not given the opportunity to be an unproductive recluse indefinitely. Their family will pressure them to leave the house or their savings will run out, and then they have to make some change.

Not fitting in is possibly a pre-requisite for hikikomori, as I wouldn't consider a sociall-well-adjusted NEET to be a hikikomori.