Yes, that's the reason everyone seems to be doing it. The other one is making them slimmer.

I've yet to take a tablet for a swim myself.

I could, maybe, be convinced having waterproofing on phones could be such an important feature that it trumps everything else, but I really doubt it's truly (as opposed to: "wow, cool, I can dive to 20000ft and still take a selfie. Best. Phone. Evar.") important to that many people.

The slimness appears to be a good marketing gimmick. And, yes, "pocketability" is important.