I'll agree that a single person consultancy is 0 risk. I can accept a job offer tomorrow and stop accepting new clients.
But growing your consultancy is just as risky as any other business, if not more so. You don't have a wide range of low paying clients, like a SaaS model. You have a few high paying clients. Any one of them leaving (or not paying) directly affects you and your employees.
I live in Washington DC, so I see this constantly. A large government contractor loses a re-bid on a contract, and they fire the team that was working on it. Fire them!
Not speaking for anyone else, but having to fire people every time a contract goes away sounds risky to me.