Up until Monroe, with the exception of Washington, the presidents tended to be more philosophical than practical.
Adams, Jefferson and Madison had all had an aversion to doing the necessary things to strengthen America's ability to defend itself.
For Adams, the country was still very much new and he had more pressing and basic things to put in place before he could think about warding off the great powers of Europe. The best he could really hope for was coexistence with the great powers of the world insofar as they would not invade the United States.
Jefferson's reason for not strengthening the military were more philosophical than the reasons John Adams had. He was a Republican in the old sense and believed in militias over professional standing armies. He feared that by creating a 'standing army', America would grow imperial ambitions and would come to be ruled by the Army and not the people.
Madison's reasons for not taking the necessary steps for increasing America's security position in the world earlier were more puzzling. He shared some of the philosophical sentiments with Jefferson on skepticism of a huge military, but he also was a co author of 'The Federalist', a publication that argued for the adoption of a National Constitution and pointed out the weaknesses of regional militias in times of war. The War of 1812 with Britain was plagued with 'faulty intelligence' and reports it would be a 'slam dunk', so that could be a reason resources weren't poured into strengthening the military sooner.
When Monroe came in, he had a clear vision for a national army and the ability to support America's desire to keep foreign governments out of the western hemisphere with action.