A lot of wealth has been created in the past few decades in markets that did not previously create much wealth—i.e. emerging economies—and which are not particularly attractive destinations for holding it.
Real estate in the world’s top tier cities and the stocks of the Dow Jones Industrial Average are high-quality, highly liquid assets—attractive destinations for that wealth.
One would expect greater demand to have developed over the past few decades for high-quality, highly liquid assets like American equities and real estate in the world's top cities.
Such relatively greater demand would express itself as relatively higher prices for such assets than have been seen historically.
The current high prices of such assets may not indicate a speculative bubble and instead may indicate a reality of an abundance of wealth relative to perceived safe investment opportunities.