You are incorrect. 35 out of 55 were.
http://en.wikipedia.org/wiki/Founding_Fathers_of_the_United_States
The 1787 delegates practiced a wide range of high and middle-status occupations, and many pursued more than one career simultaneously. They did not differ dramatically from the Loyalists, except they were generally younger and less senior in their professions.[8] Thirty-five were lawyers or had benefited from legal education, though not all of them relied on the profession for a livelihood. Some had also become judges.[9]
* At the time of the convention, 13 men were merchants: Blount, Broom, Clymer, Dayton, Fitzsimons, Shields, Gilman, Gorham, Langdon, Robert Morris, Pierce, Sherman, and Wilson.
* Six were major land speculators: Blount, Dayton, Fitzsimons, Gorham, Robert Morris, and Wilson.
* Eleven speculated in securities on a large scale: Bedford, Blair, Clymer, Dayton, Fitzsimons, Franklin, King, Langdon, Robert Morris, Charles Cotesworth Pinckney, and Sherman.
* Twelve owned or managed slave-operated plantations or large farms: Bassett, Blair, Blount, Butler, Carroll, Jenifer, Jefferson, Mason, Charles Pinckney, Charles Cotesworth Pinckney, Rutledge, Spaight, and Washington. Madison also owned slaves, as did Franklin, who later freed his slaves and was a key founder of the Pennsylvania Anti-Slavery Society. Alexander Hamilton was opposed to slavery and, with John Jay and other anti-slavery advocates, helped to found the first African free school in New York City. Jay helped to found the New York Manumission Society and, when he was governor of New York in 1798, signed into law the state statute ending slavery as of 1821.
* Broom and Few were small farmers.
* Eight of the men received a substantial part of their income from public office: Baldwin, Blair, Brearly, Gilman, Livingston, Madison, and Rutledge.
* Three had retired from active economic endeavors: Franklin, McHenry, and Mifflin.
* Franklin and Williamson were scientists, in addition to their other activities.
* McClurg, McHenry, and Williamson were physicians, and Johnson was a college president.