I don't think the JS devs drove it, at least not primarily. I think they're largely eager to exploit the enthusiasm of others (and probably they should be) and I've seen it happen. I do think it's peculiar to the web. Most other GUI app ecosystems are content not to invent their own visual vocabularies over and over or twist every mundane little program or UI element into an "experience", for instance—I think that has something to do with it. It's the set of norms and (self-imposed, to a significant extent) expectations of the web, of the fear of "falling behind" (see again: peacock feathers). I think the background of the Web in bespoke brochure-type graphic design and marketing has an absolute ton to do with it.

I think a lot of the cost has to do with the underlying tech not really being that great for what it's being made to do, in concert with everyone feeling the need to try to be special in even the tinies little looks and behaviors. It's also where a lot of the audience is so will tend to experience oddities like this more exaggeratedly than other fields.

Your manager or project owner or whoever says "we want a webapp" are you gonna stick your neck out and argue against it? "But it's what google's doing", "but Gartner", but whatever. It's cheaper! OK, sure, here's my bill. It's absolutely follow-the-leader behavior, damn the cliff. And it works well enough that it does get the job done, eventually. It's just not necessarily doing the users any favors, or your bottom line.

Decision makers do strange stuff all the time because they think it makes them look better—and, to the extent and in the way they expect it to, they are probably right more often than not. This is just one example. Whole world's run by guesswork, personal quirks of taste or incentive, and blame-avoidance (follow the trends), more than anything else, from what I've seen. It's just people, running around doing funny people stuff, hoping no-one calls their bluff.

[EDIT] actually, look at Java Applets and, more so, Flash. Folks hated the way lots of those were used, but the worst and least-justifiable uses of them happened anyway, for a long time, because managers wanted the bling. That was, seriously, it. And developers went "oh sweet I can add Flash 5 and Actionscript to my résumé, I've been wanting to play with that". That's exactly what happened then. This is exactly the same thing, just much bigger. And in both cases sometimes the tech was used well.