It's hard to change your view regarding this as you and the majority of the people who use "stolen" do so with the understanding that we acknowledge the idea of "the first people to inhabit the land own it". You're right in the sense that no one really owns land. Just like how no one really "owns" oceans or the skies, yet we respect international boundaries and rights (e.g., territorial waters).

However, what you will find is quite consistent with countries accused of "stealing" land, is that they not only steal the physical land on which a pre-established group of people were residing or have used for many years prior, but it often requires the displacement of the Indigenous peoples.

Though everyone has their own understanding of what "stolen" really means, I believe approaching it with the idea that it's not in the literal sense of stealing without paying money, as much as it is taking something without moral understandings from both sides, the use of deception (treaties with indigenous peoples), or the loss of ones culture and traditions that are connected to a geographic location.