The answer is because all effective advocacy is culture change. That includes culture writ small (at an agency, among a community of experts) as well as culture writ large. Everyone parachutes into the big proceedings and makes lots of noise. But the battlefield is prepared well in advance, shaped over time by repeated low level interactions.
The same is true in other areas of advocacy. Everyone wants an op ed in the NY Times or the Washington Post. But opinion pieces in a bunch of free local newspapers are important in shaping attitudes over time. The conservative movement grasped this a long time ago in its dedication to a continuous campaign around its issues (although it appears to have reached the limits of effectiveness on certain types of communication/organizational skills).
Yes, one must balance factors such as over exposure and allocation of resources, there is a difference between low-level activity and mobilization activity, and numerous other factors. I'm not talking about reasoned decisionmaking. I'm talking about the fact that too many advocates do not see the connection between consistent relatively low-effort, low-payoff efforts, movement building, and culture change.