UK land rights are much different then the US owing to their history of kings and lords. You don't pay property tax[1] on the land you hold. As a result there isn't much incentive to ever sell your land. Since other people still want to live on your land though you set up a deal so that they can build houses and live on that land, but they have to pay you rent. You are effectively a landlord in the truest sense of the word.

[1] If you rent out your land you start paying property tax, the government wants there own piece of that pie.

This is not false but the majority of people and properties are freehold, not leasehold. Around 3 million properties are leasehold, the majority are flats, with 1 million being houses. There are about 25 million residential properties overall, so leasehold is only about 12% of the market.

Plus, in the UK there are certainly liabilities for holding land that has a residential property on it. As the landowner, you'll be paying the council tax on it if you have no tenants, and with many councils now, that'll be a more expensive rate too (currently 150% of the standard rate for unoccupied property in my area). Of course, council tax isn't really a property tax, though it can act like one in some situations and basically covers similar things that US property taxes do.

I would point out that outside of London this is not exactly common.

The entire crown estate is held as a lease holding, and they have a lot of property in England. I know that in Northern Ireland there are many semi-detached houses on leasehold.