they've tried. I saw this youtube video awhile back about a man from gaza who built an inland fish farm to raise food to feed people because the fishermen were forbidden by isreal fro boing to the areas where the fish were.
Israel is the one who's going to build it. See, the approach is the following: eliminate Hamas, then start a sort of deprogramming the society, similar to what was happening in Germany, with local specifics of course. Such an approach will take years, but the goal is to have the same effects as in Germany.
It's possible to provide food, water, services while keeping a close eye on the Gaza population and ensuring the idea of peaceful cohabitation is dominant. The economy will slowly - or even not so slowly - rebuild, and that's a part of the demonstration of possible and beneficial, from some positions, approach.
It could be argued that Israelis need "deprogramming" - look at the scenes we've seen over the past few days, with hordes of Israeli civilians blocking aid from entering Gaza. Look at the torrent of vileness spewed forth online by many Israelis. Look at the Israeli Telegram groups where they share and laugh at images of dead Palestinian children (actually, don't look; it's just too much).
If Israel really wanted to wipe the Palestinians off the map they have the resources to do it. But they don't. Based on claims Hamas have made, in their founding charter and other public statements, if they had the resources to wipe the Jews off the map they would do it, without hesitation.
As I'm sure you know, the Hamas charter no longer says anything like that - it explicitly says their beef is with Zionists and their treatment of Palestinians, not Jewish people or the Jewish faith.
The only people we see killing day after day after day, without hesitation, are the IDF.
It's quite clear that Israel has been doing everything they can to render Gaza uninhabitable - senior officials have even publicly said that's their aim. Furthermore, it's clear the aim is a revenge-fueled massacre of civilians, followed by ethnic cleansing - senior officials regularly call for Gazans to be shipped out to other countries.
The Israelis might not have the appetite to administer Gaza themselves. They would want an Arab state to take it but no one wants that headache and they prefer using the Palestinian issue to bash Israel. UN, maybe?
Do you believe that people in Gaza will accept that Israel rebuilds it? And for how long? Because I already foresee a minority craving for independence against the old invaders that "enslaved us with money, first took our land, then they tried to buy us out...".
Etc.
The comparison with Germany doesn't stand. Two completely different situations, different histories, different people, different mindsets, different economies. You can't just let them rebuild it and hope that people in Gaza don't plan your destruction again and again.
They have to be religious fanatics because that’s all they have to cling to. If I’m going to fight a losing battle against a grossly superior enemy I also want to believe that I’ll end up in paradise for it.
You might note that that brand of fanaticism goes down rapidly in countries that have high standards of living.
Happy, fed, employed people do not become terrorists. They have too much to lose.