The 25th Amendment was used for very brief periods of time by both Reagan and George W. Bush when they were temporarily incapacitated for surgical procedures.
Congress only has the power to step in and decide that a President is incapable of fulfilling his duties under the 25th Amendment if there is a dispute between the President and the VP over whether or not the President is capable of discharging his duties (i.e., the President says that he is capable and the VP and Cabinet say he is not).
Impeachment is the only method for the Legislative branch to remove a President without first being requested to do so by the VP and a majority of the cabinet.
. . . which is why impeachment is actually easier than the method described under the 25th amendment.