They kind of did think they were building the ideal government, though. One of the major, under-emphasized influences on the framers (especially Jefferson) was Destutt de Tracey, who developed the term “ideology.” For Destutt de Tracey, ideology meant the formal study of ideas as scientific objects. Large parts of his writings deal with determining the scientifically optimal form of government (cf. Marx). In his correspondences with Jefferson, you can read how much the new government took from Destutt de Tracey, both in form and outlook, to the point that Jefferson felt compelled to explain deviations from the Count’s prescriptions.