There are certainly a number of factors, but the prevalence and addictiveness of entertainment technology is without a doubt a major factor.

Plenty of hikikomori in the safe, walkable Tokyo. What's keeping them inside?

>What's keeping them inside?

I would guess they've shut themselves in and away from the shame and rejection by society and have found the tiny inkling of happiness possible in that hole. And as society continues to rejects them further for being in a hole, the hole only gets deeper.

Understandable, since the minute they go outside, everything is stacked against them and even with help, they probably won't be able to compete on the same level as the normal people who didn't fall into the hole. I can't win outside, but I can be happy in my hole. Hole good, outside bad.

I'll admit I know very little of the culture in Japan so there are quite likely other factors. I am just talking from personal experience as a 2x year old in Australia. I would happily meet up with friends regularly but it never happens because they live too far away, most of us do not have cars and because most of us live with parents, meetups tend to be in the city where the only activity is drinking. Video games perhaps allow us to have fun when we might have otherwise forced ourselves to go out but I think the primary factor for me is how difficult it is.