Just think of it: today we send a spacecraft out of Earth orbit.... and that's all, no way of properly driving it; we can only use "oars" time by time for some corrective maneuvers, but the ship trip is actually total dependant on streams (gravitational rather than water, but just... "streams"). We can't turn back or even just stop, we can only go ahead. And once we arrive to our destination, all we can do to stop is... wait for natural forces to stop us (atmosphere drags against spacecraft or parachute).

This isn't entirely accurate. It's not that we can't maneuver our ship, but rather that there is typically no need for continued thrust. We COULD load up a spacecraft with a huge amount of fuel and zip and zoom around quite a bit, but it is neither economically feasible nor is it needed.

It isn't that we don't have the technology or knowledge, but rather that it is wasteful. Why not use gravity to assist us? Even modern ocean vessels use currents to assist them in order to conserve fuel and decrease the length of their journey. We're doing the same, and correcting our course where needed.