Ah, that’s the easy one. Because if they did that , planets would locate all their deliciously attackable facilities in the local equivalent of Mongolia, or some other mid-continental location that doesn’t have to concern itself much with naval attack. One of the few advantages you have in attacking from orbit is that you can hit anywhere on the planet, and it’d be a real shame to give that up.
Now this idea, I’ve got to tell you first off, I kind of love. In fact, I’m going to award you an official Thinking Like An Eldrae Engineer award for coming up with that one. 
But here’s the problem. If you look at the operating altitudes specified - 65 km/211,000’ at the low end - those are well above the sort of operating altitudes achievable for balloons. (Current record altitudes for them are 136,000’ for a crewed balloon and 174,000’ for uncrewed; it’s also around 124,000’ for an non-rocket-propelled aeroplane, and only around 86,000’ in sustained flight rather than a zoom climb.) Since the lift force you can get from a balloon depends on the density difference between the lift medium and the surrounding air –
– well, the density of the atmosphere at 211,000’ is 1.8e-4 kg/m³. Gas-phase hydrogen at equivalent pressure (0.12 mbar), a density of 9.7e-6 kg/m³ will still provide some lift - but when you actually run that through the balloon lift equation, you get a skosh under 1.7 millinewtons of lift per cubic meter of envelope. Even a pure vacuum dirigible won’t do you much better on that front - and while you might be able to make an envelope that it can lift, bearing in mind you need it to weigh no more than 180 milligrams, it’s for sure not going to be lifting anything else .
This all gets easier if you’re willing to dip lower in the atmosphere, of course, but then you take on other problems like needing to provide for greater structural stress and thermal shock on entry (which adds to your mass), and being reachable by much cheaper anti-air than would be needed to reach >200 kft, along with a proportionally less effective laser pd due to greater atmospheric dispersion/absorption, etc.
All of which is to say, alas, that much as I love the idea, I don’t think the numbers work on balance.