this is easy:
Israel has basically never been hit. There was no expectation for there to be a ground assault from Gaza. Occasionally missiles from Gaza reach the ground, and a UN observer on the Israeli side would not be able to help with that.
Gaza is dense and all the UN observers would routinely get killed in Israeli retaliations. The modus operandi from Israel is precision missile strikes on specific buildings, but all buildings are targets because Hamas uses human shields, not like there is any military specific area they could go in Gaza as its too dense and would also get bombed immediately if there was. So, this 'human shield' observation has no limits and there is no appeals process. There is generally no ground operation from Israel.
you're speaking from the future because that's all changed now.
edit: and as others pointed out, there are UN operations in other areas not related to the Gaza conflict. because there are other conflicts