People do that with physical land too... Isn't that much worst? What about sitting on gold, or Bitcoins?

There's an entire economic school of thought dedicated to railing against the specific problem of sitting on physical land just to keep it out of use until someone else pays you a higher price:

https://en.wikipedia.org/wiki/Georgism

It's fun to note the game Monopoly was designed as a teaching exercise for Georgism (it's meant to be awful; it's designed to show the unfairness of existing landholder/landlord structures), though the pack in rules (for somewhat obvious reasons) no longer include the pro-Georgism text and counter-rules that illustrate that the game is far more boring, but quicker and more fair under a Georgism-like model.

Indeed! I assume you’re referring to “The Landlord’s game” by Elizabeth Maggie? (The earlier incarnation)

Quite. The original rules document is in the public domain now and is a fascinating read of turn of last century economic thought that is still relevant today (as this larger discussion shows). Maybe someone could do a quick interesting reskin for "The Domainparker's Game".

Yes, it is bad with physical land which is why governments introduce land taxes to encourage value creating activities on land.

Gold and bitcoins are not problematic as holding on to them does not inhibit value creation.

Yes, but they do it to the exact opposite effect. Squatting property actually puts it to its intended use (living, agriculture etc.) Domain squatting is meant to prevent the domain from its intended use (access to a meaningful service). For gold and bitcoins, there is (or at least there should be) taxation to make sure you're not sitting on them.

Around the world there are actually laws regulating physical squatting [1, 2], because it seen as a socially positive outcome (compared to leaving houses/land disused). And there are also often provisions that allow you to prevent squatting [3], but you have to give something back (e.g. dirt-cheap short-term rentals).

[1] https://en.wikipedia.org/wiki/Squatting_in_the_Netherlands, [2] https://www.latimes.com/business/la-fi-associations-squatter..., [3] https://nl.wikipedia.org/wiki/Antikraak

Worth adding, at least in some countries, the laws allow physical squatters to legally take over ownership of land or property, if they can document they lived there / put it to productive use over many years (IIRC 20, in Poland), while the original owner didn't use it in any way.

EDIT:

https://en.wikipedia.org/wiki/Usucaption

https://en.wikipedia.org/wiki/Adverse_possession

I don't think the parallel to be drawn here is against physically squatting in property the occupants do not own, but with people who buy property and use it as a value sink; as seen with a sizable portion of non-domestically owned property investment in London, Vancouver and other high value regions.

Such property can be seen to sit unused by anyone, but it's lack of availability (when it could otherwise be used) contributes to the scarcity of property in that district, and thus inflation of property prices in that area (be it naturally occurring or otherwise)

Land, Gold and Bitcoins are all commodities.

I don't have to care which specific instance of them I have (mostly, land is where it gets funny but that's more secondary effects like transport hubs, taxation, regulations etc at which point it's not "land" it's a location I'm after).

A domain name? It's a unique registration on a global index. It's value is in that uniqueness.

If land isn't unique, can I have a piece of land exactly like yours (in the same location)?

Tell that to the farming industry.

(Yeah, I know it’s not central to your point.)