>Is there an in-between type of user technical enough to use a static site generator but not able to write their own? Or maybe the proliferation is only because they are both easy and fun to create?

Not sure if you mean to ask why people don't create their own static sites (without generators), or did you mean dynamic sites using PHP/Ruby/Python?

For the argument over dynamic sites:

Yes - the type that does not want to worry about security vulnerabilities.

My blog is made via a static site generator. I updated infrequently (once every so many months). I want to have it up and running for years without my intervention (i.e. maintenance).

I once had a Wordpress blog and treated it that way. It was hacked.

Then I built my own in Django. Then at some time it went down because my service provider updated Python libraries, etc.

The funny thing is: Using a static site generator is no more work, and has no fewer advantages. Why should I use a dynamic site or build my own?

For the argument against static sites, well then you'd have to maintain lots of links manually. A SSG gives you a lot of that for free. And you can use templates.