At least in London: Every employer moves to London because that's where the employees are; and every employee moves to London because that's where the employers are.

Of course I don't mean /literally/ every employer or employee. But I've certainly been offered jobs in remote locations and turned them down because the locations only had one employer who needed someone with my skill set.

Before railroads, the Erie Canal was one of the few ways to ship bulk goods over the Appalachian divide. This caused the mouth of the Hudson to be a center of trade, particularly shipping goods to Europe. To make money on the return trip, they would put passengers in steerage and so lots of immigrants landed in New York City.

Because a service and investment economy is based on proximity to those with money as opposed to proximity to factories or resources.