Aggregation effects. You're more likely to switch to a different employer if it doesn't mean moving. Startups will put up an office near a larger company to poa

Aggregation effects. You're more likely to switch to a different employer if it doesn't mean moving. Startups will put up an office near a larger company to poach their employees. Etc etc. This is how you get an industry crowded into a single city even if they're not actually transporting physical items around.