[zozbot234]

That's the problem that web directories solve. It's not that you're wrong, it's just largely orthogonal to the problem that you'd need a large crawl of the internets for, i.e. spotting sites about X niche that you wouldn't find even from other directly-related sites, and that are too obscure, new, etc. to be linked in any web directory.

[reaperducer]

That's the problem that web directories solve

Not really. A web directory is a directory of web sites. I can't search a web directory for content within the web sites, which is what a niche search engine would do.

[Beldin]

On the other hand, the niche search engine depends upon having such a web directory (the list of sites to index).

[teddyh]

Like WAIS?

https://en.wikipedia.org/wiki/Wide_area_information_server