For me lazy has nothing to do with it. Usually one or more of the following:

* You have to navigate a half dozen pages to get to what you want assuming you can navigate the maze of links.

* Site search incompletely indexes the site.

* Site search has poor ranking. Twenty pages of press releases and other crap mentioning what you want before the actual info.

* Site search doesn't handle spelling mistakes so you get no results.

* Site search doesn't handle related topics so if you search for the wrong or alternate phrase you miss what you need.

* No site search feature at all.

So yeah, search engine is my first choice.

Isn't exactly that the job of a search engine? I.e., that don't have to specify the exact site? Moreover, many searches are done be beginners who don't (yet) the best resource for what they search for.

But let's assume you use a search engine only for stuff you aren't intimately familar with - how could you trust the quality of the results? If they are so bad at the topics you know much about - why should they be better at the other topics?

Another way to look at it is people are expressing a need to have a single good search tool rather than having to learn every individual site's search tool. Site hosted search varies wildly in quality too since many don't get much direct use (perhaps largely because most people uses general search tools like Google). I know I've built up a (usually unconscious) decision making bias that "search on site always sucks" that makes me shy away from them in. It feels like a reasonably rational decision to do so vs being garden-variety lazy.