To clarify: there are only 24 hours in the day. One man can do one man-year of work in one year. That would be you, the consultant. Regardless of how many clients you sign up.
Thus the lack of leverage - there is an absolute cap on what you can bill per year.
This is exactly why I got out of consulting. I also don't like working for consulting companies (even if I am a full-time employee there). To many times, the company takes on too much work for the number of employees, and the employee ends up suffering (tons of overtime..usually unpaid, pissed off customers, etc).
One of the worst place charged companies a flat-fee per month for tech support (some had 300 or 400 employees). There were about 10 of us that had to take on everything.
It resulted in customers calling us up for things that weren't even really problems..mostly just preference (IE: location of icons on the desktop).
Thus the lack of leverage - there is an absolute cap on what you can bill per year.