And whereas pre-war long leaseholds had static ground rents, these modern 250 year leases tend to have rents which at least double every 25 years.
There is sometimes a legitimate justification for an up-market development, in that a lease makes control of undesirable changes much easier for neighbours than relying on freehold restrictive covenants. More often, though, it is to create an asset which can be sold separately, as Archie_tect says. Some professional ground rentholders make a substantial return from extracting excessive fees for various 'consents' - Google 'Simarc' for one of the worst.
Don't overlook that the freeholder of a long leasehold house usually has the right to buy the freehold.