There are soft kill mechanisms that can avoid the Kessler syndrome, at least in the beginning. But that isn't the reason why territories aren't defined beyond the atmosphere. It's easy for a plane to avoid flying over another country. But it isn't as simple for satellites to avoid flying over another nation. Orbital mechanics doesn't give you much freedom to do that. Satellites overfly hostile territories thousands of times on a daily basis. If even the countries with soft kill ASAT starts prohibiting that, then spaceflight will become untenable in less than a day. The lack of territorial claims in space is just the world nations being reasonable.