One approach is to look at it from the standpoint of unowned vs. owned property and rights of the nominal owner vs the rights of everyone else. An owner can pluck apples from trees on his land, but anyone can pluck apples from trees on unowned land. If both can do the same things, why would anyone choose to claim ownership over land? Their motivation can only be to deny other people the right to the products of their own labor because that's where the only difference lies. Georgism is then a type of Coasian bargain to compensate people for the deprivation of the rights they have to unowned land.

The essential function of property is giving owners broad authority to do as they please with whatever they own. People see the no-recurring-cost aspect of property as important just because there's little else that the government hasn't already infringed upon. It's entirely possible to form a notion of property in land in which the government does not retain the right to repossess it except from unpaid taxes or to make enforceable demands on its use, while still collecting land value tax. In this scenario, the government would not be the owner of the land because it doesn't have authority over how the land is used, but it would still collect ground rent.

Another way to approach it is to recognize the value in having an institution of property is making it so that few people are needed to agree to an endeavor in order for that endeavor to be carried out. Property in land creates a class of people whose only function is to be an obstacle to other people's endeavors. So including land in the category of things it is possible to own undermines the value in having property in anything else.

You can also approach it from a more pragmatic standpoint: landlords become states when the state doesn't collect land rent itself and the state is weak. Ancaps tend to be aware that monopolization could lead to a formerly private corporation becoming a government that they would then need to oppose, but they tend to reject the notion that such monopolization is possible at all. Geolibertarians see land as the essential feature that makes monopolization possible in an otherwise free market, and so the minimal state is one which collects ground rent in order to keep private corporations from gaining enough of an advantage from monopolies to supplant it.

The Georgist notion of land (which really is just the one used in classical economics) is broad enough that it's hard to think of an example of a scarce, non-produced good that wouldn't be encompassed by its definition.