There isn't enough time in the day to be an FTE and run a consulting company. But there is enough time to do your FTE work and then go out at night and network your ass off to get a large consulting contract of some sort. I.e. actually doing consulting work is blocking you from landing a big enough client / contract to quit your FTE.
That said, if the thought of shaking hands, giving free talks, and doing anything else to get a large client that you can be certain will actually pay their bill, then you probably shouldn't quit your FTE.
That said, if the thought of shaking hands, giving free talks, and doing anything else to get a large client that you can be certain will actually pay their bill, then you probably shouldn't quit your FTE.