And because a much larger percentage of those people want to live in big cities compared to 50 years ago.
If you look at London (since that's essentially the article), population has actually only reached back to its 1950 level.
But attractive cities can now pull people and investment from a global pool of billions of people and thus the market does its thing and prices move accordingly.
https://en.wikipedia.org/wiki/Urbanization_in_the_United_Sta...
The massive change in urbanisation is happening in developing countries. In developed countries it happened earlier and is not really a thing anymore.