Because I don't want to fork out for the servers and lines necessary for the speed I need. The leased line is six figures a year alone, a direct connection to the hub would be a bank breaker solo, you get economies of scale on things like cqg, ISVs reuters, bloomberg, squawks and most of all round trip rates.
I have 2 IT gurus at my beck and call, a risk manager to broke me out should things go wrong, a hub connection, as well as VPN and leased line alternatives. The social aspect of not being hermit loser in your own flat... so many reasons. I can't be arsed to go and buy umpteen computers, await delivery, power backup units, 9 screens, brackets to hold them and then spend hours on tech support lines setting up my own accounts being interrupted by phonecalls, having to do admin for those services, back office queries etc etc.
The office here facilitates setting up your Dubai company with the 50yr tax free guarantee as they themselves are half owned by the government. One individual german trader set up his own office and is still not incorporated in the tax free zone after 19 months. I was formally registered in 3 weeks.