No doubt city centres are walkable but this comment begs the question though - why are city centres 2x as expensive as suburbs? If the density and land scarcity

No doubt city centres are walkable but this comment begs the question though - why are city centres 2x as expensive as suburbs? If the density and land scarcity is the reason then why do developers want to build on more expensive ground in the city than in the suburbs? Why are people willing to pay 2x to live in cities than in suburbs? I think the real reason is that walkability (and hence city centres) are desirable which makes them cost more, and not because city centres are ipso facto expensive.