The problem with that is that targeting may be much more difficult. A passive weapon could be thrown off target over long distances due to gravitational perturb

The problem with that is that targeting may be much more difficult. A passive weapon could be thrown off target over long distances due to gravitational perturbations or survey inaccuracies, and would likely have been mistargeted in the first place due to civilizational changes at the target (kinda like the problem with aiming flak, your target can move unpredictably between when you fire your gun and when your shell arrives, see https://www.youtube.com/watch?v=eRJC2mkrZks https://www.youtube.com/watch?v=eRJC2mkrZks). An active weapon could be targeted at a solar system too far away for an accurate preparatory survey, then identify and target rocky planets and large artificial structures once it arrives. It could also perform course-corrections in transit, and remain substantially dormant for much of the trip for stealth and surprise.