But it kind of is. It's like the nuclear bomb. It changes EVERYTHING. Once it's on the table, the entire world must find a way to self-control it's entire civilization to not destroy itself.We know that Warp Travel doesn't stop bellicose polities from not being bellicose, eg they can get warp and go on a rampage. Klingons and Romulans and many smaller species come to mind. It's not a lithmus that 'this species survived! Yey!', it's 'Well, they can visit you and you can visit them so the gig is up'.
How this works when, say, you might have a pre-warp interstellar civilization (if say, Humans got to Alpha Centauri and have a colony there, or other pre-warp but transtellar species) is unknown. Maybe warp drives act as a sort of cover to 'real space' sensors - you can't see the ship at warp, so pre-warp societies probably wave it off as subspace or gravimetric oddities and hopefully can't trace or track them enough to realize they're a pattern and the PD is just 'don't pop up in front of them'?
The Klingons also seem to violate the prime directive, and Q alone knows how many species both of them, or the Cardassians, Breen, Tholians, whoever, have rolled over - the only limiter being, to them, probably being 'Will the Federation know about it/care enough to make it a crisis'.
It isn't a GUARANTEE - but the chances are so much higher. Just imagine someone giving the Aztec Empire, or Spaniard conquerors nuclear weapons. Modern nation states are far from perfect. But they've developed in tandem with technology enough to "live with" such dangerous technologies.
A warp drive would change the world similar drastically.
As for Klingons, Cardassian etc. breaking the prime directive? That doesn't rule out they still have it. All modern Nations part of UN have promised to not start wars of aggression. And that still happens often enough.
However - these are more the exceptions. We don't live in a colonial world anymore. And the Klingons are conquerors. But they don't conquer always, and everything.