All generation and consumption have to be almost perfectly balanced every second of every day. And the power company doesn't have good addressability of load. Worse when you restore power to an area all their stuff turns on in parallel giving an inrush that could be 3x or more the steady state.
A black start is a very drawn out process of bringing generation and load online in a balanced way and with wait times between load increases for stabilization.
That's probably the reason most grid black start facilities in my country (Brazil) are hydroelectric dams, which need none of these.