Why is the presidency viewed as weak prior to TR?

[Sir Mohamed]

It wasn't always considered weak before TR, Lincoln for sure greatly expanded the power of the presidency. After the Civil War, congress already went back stripping powers from the office because of Andrew Johnson's hostile actions and total lack of competence. Gilded age presidents were also considered weak since, during peace and relative stability, political initiatives came less from the executive branch and more from within congress (as initially intended by the founding fathers). The prez was kind of left with approving federal legislation and overseeing its implementation. That was also the case between Jackson and Lincoln, when all the WH occupants of that era lacked leadership skills and didn't have much political capital to begin with while the union was tearing apart.

FDR, obviously, massively expanded executive powers and his successors continued on that path. By the late 60s, political scientists spoke of the "imperial presidency", which ultimately resulted in Watergate and many abuses of powers. In its aftermath, congress passed some laws to restrict executive powers, like Congressional Budget and Impoundment Control Act of 1974, though all these measures failed to permanently reduce the presidency's powers. In recent years, executive powergrabs and overreach continued as a divided congress often failed to deliver major reforms or pass the president's policy goals.