Why we might be alone in the Universe

[Anomalocaris]

Why does the Prime Directive apply at all?

It does not apply.  Like much of Star Trek it is a conceited delusion about the degree of convergence possible between the dictates of its moral aesthetics and those of tge most powerful motivations of our specie.

But interference does not necessarily serve any useful purpose commensurate with the trouble,  So lack of interference does not imply any sort of equivalent of the prime directive