What would the point be? Israel has never abandoned Gaza. It has ceased to patrol its cities — a dangerous activity — in favor of building a wall around it and patroling from the fence, which is far safer and cheaper. Selling this tactical reorgonization as a good-will gesture towards the Palestinians is one of the most successful political maneuvers Israel has been able to pull, diplomatically speaking.
Israel still controls all of Gaza's borders — even those with Egypt, via collaboration. It has a naval blockade on its coast. It controls its airspace. Everything that goes in or out of it. Most money transactions and fluxes in or out of Gaza. This is not going to change by a declaration of independence. Could this new country you are proposing, establish its own diplomatic relationships and trade agreements with other countries? Could it buy defense material from Iran, Russia or China to equip its newly created armed forces? Of course not, so what would be the point of it? Its only possible consequences are that Israel would sell it to the world as proof that the Palestinians renounce any territorial aspirations to the rest of Israel, and splitting the Palestinian population (yet more) between Gaza and the West Bank.
As always with questions related to the Palestinian-Israel conflict, this is turning to a flame war more than a QA, but I'll add more info, as unbiased as I can find it (although it will be deemed biased anyway) about the points I'm making.
First, the issue of the reasons for the Gaza withdrawal (1).
Even current center-left opposition leader Isaac Herzog, who supported the withdrawal at the time, finds it troubling now. Demographically, he says, it was the right thing to do, but “without a doubt, from a security perspective, the disengagement was a mistake,” he told a right-wing conference last month.
What did he mean with "demographically"? Well, as you can read here (2), the main concern was that Palestinians are slowly changing its historical demands from asking for a Palestinian state - which is every day less likely to ever be, and if it was, it would be unsustainable - to asking being allowed to vote as israelian citizens, denouncing Israel as an apartheid state. This accusation was deemed far more probable to gain traction among the rest of the world - specially if accompanied with an end of violence from the Palestinian side - and could even sway the USA, if not in favour, at least to a much more neutral position.
Actually, nor more-or-less neutral (3) neither even in unashamedly pro-Israel sources (4), no mention of Palestinian welfare is ever mentioned among the reasons that caused the unilateral israelian withdrawal from Gaza.
It views the disengagement as a cunning tactical maneuver designed to shore up Sharon's domestic popularity, deflect international pressure from Israel, and forestall any further withdrawals from the West Bank. 3
With security breaches increasing beginning in 2000 and an escalation of attacks on the Israeli civilian population emanating from the Gaza Strip,1 the Israeli Cabinet decided in 2004 to disengage from Gaza with the specific intent to no longer occupy the territory. 4
Besides the demographical time-bomb, there were security and economic reasons for the withdrawal:
As a consequence of the attacks that have emanated from Gaza, Israel withdrew from the territory in order to end its legal obligations as an occupier of Gaza. 4
By giving up the Gaza Strip and certain parts of the West Bank, the ruling Likud Party instead shifted focus toward pre-serving Israel’s Jewish identity, democracy, and security. At the time of the withdrawal, only 8,500 Israeli settlers lived in Gaza, among a pop-ulation of 1.375 million Palestinians, but they required about 3,000 soldiers, a substantial monetary cost and potential source of risk. 2
About the effective control of Israel over Gaza, well, you have 4 entirely devoted to try to debunk the official position of the UN:
Secretary General Ban Ki-Moon declaring in January 2009 that “the U.N. defines Gaza, the West Bank, and East Jerusalem as occupied Palestinian territory. No, that definition hasn’t changed." 4
But of course, you can just google "Gaza open air prison". You'll find a lot of references from pro-Palestinian sources (5):
“One Israeli analyst has compared the proposed solution to transforming a Third World prison into a modern US super-max incarceration facility. The more civilized exterior will simply obscure its real purpose: not to make life better for the Palestinian inmates, but to offer greater security to the Israeli guards.” 5
But you can find the terminology discussed even on The Washington Post (paywalled).
And that's all I'm going to add, since discussions about the Israel-Palestine conflict are as unsolvable, apparently, as the conflict itself.