I think it makes sense from a business perspective on Samsung's part. As an OEM that's in direct competition with Motorola, LG, Sony, or HTC they have no incentive to make a product that enables people to buy anything other than a Samsung product.
Like you I wish they would have considered making it compatible with other OEM's so people like me wouldn't be so limited in our device choices. But Samsung is banking that their smartwatch is such a superior value over the other alternatives that people will buy into their ecosystem and thus not just sell watches, but more and more smartphones along the way.
It's a gamble on Samsung's part for sure, but with their resources they are probably the only OEM that could afford such a gamble.
Sony makes a respectable smartwatch, compatible with with other Android devices, but without the resources and marketing power of Samsung the product has never, and never will, gain mainstream acceptance.
If the other OEM's could do what Samsung is doing I believe they would try it also.
Manufacturers make products, not out of the goodness of their heart, but what they believe will sell and increase their market share.
I dislike that my other devices have been rendered useless as of this point, but unless an OEM makes a smartwatch better than the Gear then I'm sticking, at least for now, with what I have.