Because look at the many times that the Torah mentions Philistines and the Land of the Philistines!
It's been a while since Palestinians claimed to be Philistines. It was a common claim at the UN in the 1940s, when the Syrian representative stated, "The Palestinian Arabs are the descendants of the same inhabitants of that country of forty centuries ago who fought in the first campaign which the Jews waged against Palestine in the fifteenth century before Christ. In the Bible, they are called the Philistines. After about the thirteenth century, they adopted the Arabic language, which was later replaced by the Syrian language, a language closely related to the former. These people have not changed. They are the same people who were living there then. They have been there for forty centuries--since prehistoric times."
(Note his backhanded way of saying that Palestine is really a part of Greater Syria.)
The claim that Palestinians were Syrians became less popular as the term "philistine" in English is equated with narrow-mindedness and being uncultured.
The obvious next move was for Palestinians to claim to be Canaanites, because that way they can say that the Jews ethically cleansed them twice, and Canaanites were definitely in the bulk of the Land of Israel before the Jews.
But when Jerusalem became an issue, Palestinians suddenly became Jebusites, a people who have no independent known history outside the Torah. But they lived around Jerusalem, so therefore Palestinians must have been Jebusites.
Of course, the same Bible that says Jebusites existed says that they legally sold Jerusalem to King David.
Notice that there is never the slightest bit of historical, scientific, linguistic or cultural evidence for these contradictory and supposedly unbroken histories of Palestinians.
So now we are back to Philistines, since that is the most convenient lie for the current threat.
Any lie will do, as long as they counter Jews' claims.
Which reminds me of a recent comic of mine: