Totally different technology, different support, different training, different mission. You might as well have said why not the post office?
Having worked in the space industry for a long time, and having an aerospace engineering degree (in which you must choose whether you're specializing in air or space, because they diverge in a big way), I think it makes sense. It makes more sense than the coast guard, which should probably be navy. If we're worried about spending, let's roll the coast guard into the navy so that we have a conservation of branches of armed forces.
My real issue with the space force is that it's not the new branch of the military I'd have created first. The new branch I'd have created first is the cyber force, for dealing with international hacking. Especially in light of foreign governments trying to hack our elections, I'd think this would make a huge amount of sense, and was more pressing. Since we're conserving branches of armed forces, I'd say let's roll the marines into the army or navy to compensate. I'll say navy because it's in the name marines, but I just rolled Coast Guard into Navy so let's roll Marines into the army.
So here's my proposal:
Navy/Coast Guard -> Navy Force
Army / Marines -> Army Force
Air Force
Space Force
Cyber Force