I think one reason is the the rich want to find a city they want to own. Countries like the US have a number of large or mega cities. But for smaller european countries, there is one megacity for each country, usually the political and financial capital is also the same in such cases e.g london in uk, paris in france. And most megacities will be close to sea or have a port near them.
Historically if you see, the rich people were the merchants who did global trade. They settled close to sea ports and most of the rich network built around those cities. Then as we moved to mordern times, more people started moving to these cities because of better employment opportunities. And people keep moving there because the rich never move. They keep occupying a small section of the city, for example manhattam in new york. Moreover, having an office in these areas, where real estate has gone through the roof is not only a status symbol but also the place is ripe for business.