Work it out, when did the Palestinians leave Israel/Palestine? Late forties, early fifties, the youngest genuine refugee would now be approaching sixty.
Do you suppose they gave up their land and livelihoods voluntarily, in order to go and live in a refugee camp in Syria or Jordan? Do you imagine they had a choice?
Admittedly, it is fairly impractical now for the refugees to return - that would mean a defacto end to Israel as a "democratic" state. However, it doesn't diminish the injustice that was served upon them when they were expelled.
Winston Churchill was correct when he opposed the creation of the state of Israel, on the grounds that it would cause a century of wars. His solution was to reintigrate the displaced jews back into their countries of recent origin in Europe - as ever he was right. Unfortunately Roosevelt overruled him with today's disastrous consequences.