Beyond "it showed up on a popular television show", there is very little reason to believe in the Prime Directive being a real thing. It already appeals only to a particular subset of humanity, and there's little reason to believe it would appeal to all aliens. And there's a lot of humans who would consider it actively immoral, too.
It's one of those Fermi paradox answers that requires all of the supposedly millions or billions of intelligent species to all happen to have adopted one particular answer uniformly and for all time. Those answers aren't very compelling without a rock-hard explanation as for why it's absolutely mandatory that all star-faring civilizations will A: adopt that ideal immediately and B: keep that ideal intact for millions or billions of years despite all the changes they may go through.
The Prime Directive was a tool for creating drama, not a well-thought-out philosophical statement on the inevitably of certain ethics. Heck even in Star Trek itself, only the Federation has this policy, which it violates accidentally and otherwise fairly often anyhow. The other major civilizations show no sign of having this policy at all.
It is also an easy answer to try to explain the Prime Directive in a dramatic setting in such a way that our real world might still be in a space full of aliens, despite the fact that if space was really crawling with aliens there shouldn't be any such thing as virgin territory anymore and everywhere in the galaxy should be showing millions of years of proof of alien interactions, just as it's increasingly hard to find a square meter of Earth's surface that doesn't have some sort of unambiguous evidence of the presence of a technological civilization, if examined with powerful enough tools.
It's one of those Fermi paradox answers that requires all of the supposedly millions or billions of intelligent species to all happen to have adopted one particular answer uniformly and for all time. Those answers aren't very compelling without a rock-hard explanation as for why it's absolutely mandatory that all star-faring civilizations will A: adopt that ideal immediately and B: keep that ideal intact for millions or billions of years despite all the changes they may go through.
The Prime Directive was a tool for creating drama, not a well-thought-out philosophical statement on the inevitably of certain ethics. Heck even in Star Trek itself, only the Federation has this policy, which it violates accidentally and otherwise fairly often anyhow. The other major civilizations show no sign of having this policy at all.
It is also an easy answer to try to explain the Prime Directive in a dramatic setting in such a way that our real world might still be in a space full of aliens, despite the fact that if space was really crawling with aliens there shouldn't be any such thing as virgin territory anymore and everywhere in the galaxy should be showing millions of years of proof of alien interactions, just as it's increasingly hard to find a square meter of Earth's surface that doesn't have some sort of unambiguous evidence of the presence of a technological civilization, if examined with powerful enough tools.