Letting Hamas have control of Gaza created a territory controlled by Hamas that produces weapons to the detriment of its people, that aims weapons at cities anywhere near it, and who teach violent extremism in their united nations funded schools (where hamas keeps winning elections for leadership too).
Many people in Israel will not accept a similar situation for the west bank, and will vehemently reject leftists without actual solutions. The settlements are a different faction than people who might vote for leftists if they had demonstrable solutions.
But that's a justification for creating settlements anywhere Hamas is, which is all of Palestine.
"We must build settlements on contested land, because Hamas is building rockets, because we are on contested land, because we must build settlements on contested land, because Hamas is building rockets..."
Again the faction that is pro settlement is not the kind that will vote in realistic left wing governments if they appear. The issue is not just that faction exists it is that there is no solution from the left in Israel that stands basic scrutiny for centrist to think about voting them in.
If there is no long term solution in view then people will act now to put themselves in the strongest position if something changes. Land swaps in future deals etc are only made stronger, and the faction not interested in giving it away are appeased.
Also Hamas has political control of Gaza but not the west bank. There will be no future where Israel accepts Hamas control of the west bank. If you cannot integrate this into your thinking then think about the US making peace with Isis, etc because some Syrians picked them.
Many people in Israel will not accept a similar situation for the west bank, and will vehemently reject leftists without actual solutions. The settlements are a different faction than people who might vote for leftists if they had demonstrable solutions.