And you know this for a fact, how?
Compare the "resumes" of the other SCOTUS judges that hadn't previous served as judge to Miers resume. Examples:
Louis Brandeis
Entered Harvard, graduating from its law school in 1877 at the head of his class.
Most successful attorney of his day in Boston, achieving financial success and taking an active role in progressive causes.
In the 1908 case Muller v. Oregon, Brandeis, acting as a litigator, established what became known as the "Brandeis Brief", the first instance in the United States that social science had been used in law and changed the direction of the Supreme Court and of U.S. law. The Brandeis Brief became the model for future Supreme Court presentations.
Felix Frankfurter
Graduated from City College of New York.
Received law license from Harvard Law School in 1902
Named assistant of New York attorney Henry Stimson, and continued when Stimson was named Secretary of War Helped found the American Civil Liberties Union in 1920
Provided legal research for the the case Nicola Sacco and Bartolomeo Vanzetti
Published major law books including The Business of the Supreme Court (1927), Justice Holmes and the Supreme Court (1938), The Case of Sacco and Vanzetti (1954) and Felix Frankfurter Reminisces (1960).
Contempories considered him the nation's preeminent scholar on labor law
From 1914 onward, popular professor at Harvard Law School.
Lewis Powell
Attended Washington and Lee University, garnering both an undergraduate and a law degree from that university, and elected president of student body as an undergraduate.
Attended Harvard Law School for a master's degree.
Rose to the rank of Colonel in World War II, working in intelligence and decoding.
Partner for over a quarter of a century at Hunton, Williams, Gay, Powell and Gibson, one of the largest law firms in Virginia.
From 1952 to 1961, he was Chairman of the Richmond School Board. Powell presided over the school board at a time during the turmoilous time when the the State of Virginia was locked in a fight over Brown v. Board of Education. The Richmond School Board had no authority at the time to force integration.
Served as President of the American Bar Association during the year 1964-1965. Involved in the development of Colonial Williamsburg, where he was both a trustee and general counsel.
In 1971, he wrote the famous Powell Memorandum to a friend at the U.S. Chamber of Commerce. The memo may have sparked the formation of one or more influential conservative think tanks.
William H. Rehnquist
Attended Kenyon College for one quarter in the fall of 1942, before entering the U.S. Army Air Force
After the war ended, Rehnquist attended Stanford University with assistance under the provisions of the G.I. Bill.
In 1948, he received a bachelor's degree and a master's degree in political science.
In 1950, he went to Harvard University, where he received a master's degree in government.
Rehnquist graduated first in his class, probably based on the fact that Rehnquist was class valedictorian during graduation ceremonies.
Worked as a law clerk for U.S. Supreme Court Justice Robert H. Jackson from 1951–1952
Served as a legal advisor to Barry Goldwater's 1964 presidential campaign.
Served as Assistant Attorney General of the United States from 1969 to 1971.
Headed the U.S. Justice Dept. Office of Legal Counsel
Served as the chief lawyer to Attorney General John Mitchell
Harriet Miers
Graduated from Southern Methodist University with her bachelor's degree in mathematics (1967) and a Juris Doctor degree (1970)
Cerked for Belli, Ashe, Ellison, Choulos & Lieff in the summer of 1969
Worked in private practice for the Dallas firm of Locke, Liddell & Sapp (and predecessor firms prior to mergers) from 1972 until 2001.
In 1989, she was elected to one two-year term as an at-large member on the Dallas City Council; she declined to run for reelection in 1991
In 1995, George W. Bush, then Texas governor, appointed Miers to chair the Texas Lottery Commission.
In November 2004, President George W. Bush named her to succeed Alberto Gonzales to the post of White House Counsel