> When a human first laid claim on a stretch of land, you could argue that this in itself constituted theft, at this very moment this person deprived every other human being from accessing or cultivating this land. What moral right did this person have to claim this land?
In many countries, including Switzerland, there are of course specific laws that recognizes that property rights does not extend to full monopoly rights on land use.
I'm from Norway, and the Nordic countries in particular have very extensive exemptions to private property rights that recognises that there are certain rights to land use, as a limited and fundamental resource, that are held in common irrespective of property rights.
Sweden has this baked into the constitution. Norway until the late 50's I think did not have a law mentioning this right as it was considered so fundamental that it was long considered unnecessary to codify and courts would apply the principles anyway (this may not sound unusual from a common law perspective, but Norwegian law is a codified system of the Scandinavian model that evolved from Germanic law with Napoleonic influences, not at all a common law system) - the way you don't specifically cover the right to freely breathe the air.
In Norway (and it's much the same in Sweden and Finland), this e.g. means that outside of urban settlements you don't need to ask permission to walk through a privately owned forest, or across a field, for example (though you may not walk across cultivated fields other than when frozen). Even if fenced in. Nor is the land owner entitled to stop you or erect barriers to prevent you access (that does not mean barriers are illegal if they have reasonable purposes, such as e.g. keep wild animals out, but there'd be expected to be gates etc. allowing access). You can pick berries or mushrooms etc. for your own consumption but not hunt without permission. You can camp. There are limits, but they are in line with what you would expect of considerate campers. It is also generally considered rude to e.g. camp right outside someone's garden without introducing yourself and asking, even if on land you may technically have right to camp on. There is a general zone of 100m along the shoreline that it is not normally permtted to build on, even if part of your property (though exemptions are granted, with the caveat that it is still not permitted to prevent access or usage of the immediate shoreline no matter how close you've been permitted to build).
This is considered so ingrained in our culture that it is (or at least was when I was a kid) taught repeatedly in primary school.
Fencing off or otherwise trying to restrict access to "utmark" ("outlying land"), even if it is your own property, is not just legally restricted but considered reprehensible and morally indefensible to the point where it regularly causes major outrage if someone is trying to push the limits (or e.g. if particularly extensive exemptions are given to build near the shore).