Because people don't keep quarantine
If you have most people vaccinated and empty hospitals, that's not such a big deal. If you have lots of vulnerable people and full hospitals it is.
Not sure why this is so hard to understand
So let me understand what you're saying:
No vaccination, hospitals aren't empty, lots of vulnerable people and full hospital - policy is: We
allow Israeli citizens that have been documented as infected and recovered in Israel not to quarantine upon return to Israel or exposure to confirmed COVID-19 case. If we don't have domestic documentation, we'll accept only a foreign positive PCR test (which we cannot verify) and a foreign negative PCR test (which we also can't verify) and not require a PCR test upon arrival to exempt from quarantine.
We
won't allow ANY antibody test, whether foreign or domestic as proof of prior infection. If you don't have a positive PCR test, you can't be considered recovered and are therefore subject to mandatory quarantine (which we don't really enforce that much, but if you try to leave the country while you're supposed to be in quarantine, you won't be able to, and we will hit you with a ILS 5,000 fine).
Very high vaccination rate, more than half a million recoverees in Israel, no hospital overcrowding - policy is:PCR test required no more than 72 hours prior to departure from foreign origin, PCR test required upon arrival (at least you don't get charged for that one). If you recovered from COVID-19 overseas you can be exempted from quarantine until 6/30/2021 as long as you produce an Israeli positive antibody test (for which you are allowed to exit quarantine - we won't allow for those or vaccines administered at the airport under any circumstances, though the well connected know how to arrange for those privately), which needs to be sumbitted online (good luck if you don't have easy online access) and when someone has a chance to look at your request, you will be exempted from current and future quarantine through 6/30/2021, regardless of when your recovered, how high your antibodies are, or anything else.
This makes perfect sense, doesn't it?