Ask HN: How important is it for a site to have its own custom search?

The link goes to a quick site a reddit user created to "solve" the persistent meme of "reddit search sucks". This brings up a larger question as to why so many companies seem to want to control search on their site with custom rolled search systems.

The question about this is, why doesn't reddit just do this? Why don't other sites do this?

It's free, the search quality is very good, and there is no cost for infrastructure, DBAs, developers, etc.