It's remarkable how often they come up in topics around FR. :^) They didn't exist per se. The Sea Peoples, despite having been mighty enough to blot out empires throughout the Aegean, Anatolia, Near East, and northern Africa, built no cities, left no characteristic graves, weapons, inscriptions, or even rulers over the vanquished -- and most peculiarly of all, left no shipwrecks anywhere. They were invented out of just two references in the late New Kingdom period in Egypt in order to try to make sense of out the mess created by proponents of the conventional pseudochronology.
That's one way of looking at it. Another is that they did and they did leave traces which we attribute to another culture of which we don't really know the origins (which is in perhaps what became of the Sea Peoples - they bacame a culture we do know). Probably no way to ever really know.
They may indeed be the "dark matter" of archaeology, the mysterious invisible force that we don't understand but solves problems with existing theories. But I'm less certain than you that they are a figment of the academic mind.