Mainly because all the productive land was taken from them.

The few farms and fisheries in Gaza can't produce enough for 2 million people. Especially since they aren't allowed to mechanize their agriculture, and are limited in maritime range for fishing.

And forget the possibility of trade ports...

I'm not calling you names, I'm starting a plain fact that you are an ignorant person. That's what the word ignoramus means. Feel free to educate yourself https://www.palestineportal.org/learn-teach/israelpalestine-the-basics/maps/maps-loss-of-land/