I believe part of it is the cost. It used to be too expensive to get something like a decent commercial 3D printer (though at the same time). Now you can get one for less than the cost of RepRap components back in the day, so it is stupid to do it if you just want to print stuff out (it might even be more expensive, and will likely produce worse prints).

If, however, you want to learn how to build one, or just have an experience of building your own tool you'll use to create stuff, it is most definitely not stupid to do it.

With 3D printing becoming mainstream, you'll have fewer people in those communities who support and live that approach though, and you'll need to look harder to find documentation and like-minded people.