Because you need coordination and organization. If you don’t reach a critical mass of participation then the handful of people involved will just be handled as if they were skipping work for any other reason. And if it only lasts a day it can simply be waited out.

Strikes require funds, funds require dues, dues require unions. You have to ensure that striking workers will have some form of security in order to reach a critical mass. People have families to feed. Striking without funds or organization could only work if everyone was really dedicated to the cause, and if that was the case, then what’s stopping them from forming a union and collecting dues in order to do it right?

You can’t lay down the sort of moral “line in the sand” that you need if participating means, “I didn’t show up to work, all of my coworkers did, and now I’m unemployed and will be on the streets if I can’t find a way to make rent soon.” These “General Strike Now!” calls happen basically every other week, with no coordination or thought of strategy.

Strikes involve inherent individual risk for a collective benefit. It’s a collective action problem, which can only be overcome by an organized structure mitigating the risks and inspiring confidence in the outcome. A “wildcat general strike” isn’t really a thing.