From the TNG Technical Manual (for the Galaxy class, but one can safely assume operations haven’t changed that much):

In the event a deuterium tanker cannot reach a Galaxy class starship, the capability exists to pull low-grade matter from the interstellar medium through a series of specialized high-energy magnetic coils known collectively as a Bussard ramscoop. Named for the twentieth-century physicist and mathematician Robert W. Bussard, the ramscoop emanates directional ionizing radiation and a shaped magnetic field to attract and compress the tenuous gas found within the Milky Way galaxy. From this gas, which possesses an average density of one atom per cubic centimeter, may be distilled small amounts of deuterium for contingency replenishment of the matter supply. At high relativistic speeds, this gas accumulation can be appreciable, though the technique is not recommended for long periods for time-dilation reasons (See: 6.2). At warp velocities, however, extended emergency supplies can be gathered.

[my emphasis]

In those three places there are the qualifiers “in the event…”, “contingency” and “emergency”, which indicate that the Bussard collectors are only activated when needed and are not always on.

The reason is simple: the amount of deuterium that can be gathered is usually in negligible amounts unless you’re in proximity to a dense source of the element, like in a nebula. So it’s just not energy efficient to keep the collectors on all the time.