I think thats a vBulletin thing, and not necessarily a site mandate thing.

I'm on several forums around the web, and few, if any have a search facility that works well for me. The problem with google is how often it might scrape the site; more valuable sites get updated more frequently, while on others, you may miss fresher results.

I'm guessing it's a resource issue; I don't know how much might be eaten up the fewer characters involved.