Why are strikes by large numbers of people accepted as normal tactics in employment disputes, when an individual striking alone for better conditions would probably just be fired for not showing up?

I understand unions have collective power, and that withdrawal of labor en masse is a key tactic to make employers listen.

But what is the delta between single individual taking that action and that collective action to make one an accepted "withdrawal of labor" while the other is basically giving your employer grounds to fire you?

Is it allowed to just "withdraw" your labor at any time, or is that just something where the sheer scale of the workforce represented by a union makes it "right"?