Because aside from your cultural biases to slap your cocks against each other for what I assume is no good reason: submarines go UNDER THE WATER and space ships go INTO SPACE which is not only a major tactical advantage to control >99% of the known universe, (I'll have you know Russia controls way less than half of the known universe,) but also a submarine regardless of how well-stocked it is in Tomahawks or swarthy seamen will provide little resistance against a stealth bomber, orbital drone strikes, or if we take it to its logical extreme: the death star.
Now obviously you don't actually think that "Sea, space, what's the difference? they're both tin cans, right?" because that'd be LUDICROUS. But even if you are just weighting in on domestic military matters, which we are all qualified to do after all, especially us foreigners, I think the US has more than a big enough navy to afford to spend a little more on not-the-navy.
And let us all just agree that a submarine and a space shuttle is principally the same but not functionally, and trying to retrofit the SSBN with thrusters probably wont be very successful. I am glad we had this conversation and can now part as friends.