Why does a sovereign have unconditional ownership in perpetuity of a real asset? Nobody owns land by virtue of being on it or saying they own it. Another sovere

Why does a sovereign have unconditional ownership in perpetuity of a real asset? Nobody owns land by virtue of being on it or saying they own it. Another sovereign making a claim to the land is valid and puts ownership in dispute. The 2 parties have multiple ways to resolve the dispute with war being a mechanism if others paths are unsuccessful. Yes, wars of conquest are "illegal" today but that's not a real thing - it's just a declaration by some group of humans who wish it to be. They certainly weren't "illegal" in previous times so any land acquired then is clearly not "stolen".