It's less about specific skills (e.g. writing or accounting) being in demand in those places and nowhere else, and more about what kinds of companies are demanding those skills and how much they're willing to pay. Like legodt mentioned in a sibling comment, the most influential and prestigious companies in a lot of industries are pretty concentrated in a handful of big cities.

It's not just that different industries have their hubs - media, design, finance in NYC, entertainment in LA, tech in SF. A whole ecosystem of supporting firms grows around these companies. So the most influential and prestigious law, accounting, and consulting firms tend to cluster in these cities too.

There also are a lot of people who don't necessarily want to spend their entire lives in, say, NYC but think it will be a good career move to get a prestigious name on their resume before going somewhere else.

At this point I think there's kind of a chicken and egg situation where a lot of ambitious, talented people flock to major cities because firms are there, and firms stay planted in these cities because so many talented, ambitious people are flocking there.

Anything in fashion, design, media, etc. Fields that have global distribution for creative work at large scale is what I'm familiar with and unless you are willing to work for a smaller company (I.e. not large consultancy or high profile work), you have to be in a major urban center.