Part 1: Probably because vanilla religions were already working as intended - if it ain't broke, don't fix it. (We've all seen too many bugs introduced by "improving" something...)
Part 2: Much as we would like to think otherwise, [any company] is out to sell their own products, not someone else's. If it's modable, that's a free bonus or an unfortunate consequence, depending on the company. (Even though we don't charge for mods, they're still our products, not PDX')
(I don't like it either, I'm just slightly more cynical and realistic :/)