Thought by whom? It’s one theory, but not the most common recent one, where they are variously generic Sea Peoples (Peleset) conquered and settled in Canaan by the Egyptians, or Luwians (Anatolians), or even (but less likely) just “Ageanized” locals heavily influenced by what was left of Mycenean culture. The Pelasgian->Philistine theory was a current one … around 1900, I think. Not sure if it has as much currency today, to justify a blanket “they are now thought to be”, maybe give us a cite?
Also note that “Pelasgoi” refers to all pre-Mycenean inhabitants of what became Greek territory, and also some neighbouring territories, not specifically Cretans.
Current theory also somewhat favours the idea that the bulk of the Philistine kingdoms were indigenes, and only the elite stratum was of an origin outside Canaan.