Perhaps because, for the kit (and the kit manufacturer / retailer), pitching dry results in a great brewing experience. The process of pitching dry appears to be simple, effective, and yields a good tasting beer. No worries (or support calls) around killing the yeast by putting it into water that's too hot to re-hydrate it.
I realize that many (perhaps the majority, maybe all )of the forum admins and really long time posters here are strongly in favor of re-hydrating (for many reasons). And I'll accept that, for them, re-hydrating is the only way to brew well. I pitch dry, most of the time, with good results. On occasion, I try re-hydrating, but at the moment, I'm not seeing a difference for the way that I brew.