it is accepted by the UN because there is no other figure. they have no other way of estimating, also the UN is far from a neutral element in this conflict
The UN is a political body composed of the political interests of its members, which are mostly authoritarian states, and it hasn’t shown much support for Israel due to the vast membership of Islamic countries. Parody case in point, Iran being Human Rights commission seat
UNRWA, the UN agency on the ground was shown again and again to be in the very least in the mercy of Hamas therefore cooperative, at most its infrastructure and staff was used by the organizations for attacks against Israel and to hold hostages.
The ICJ in this case heavily quoted UNRWA as a source while it is an extremely problematic one.
The hospital bombing I quoted above is an example case where many experts tried to estimate casualties based on evidence, they arrived to figures that range from tenth to fifth of what Hamas published.
This together with the fact they control the casualty figure and have a clear interest at inflating it in order to stop Israel from attacking, is pretty obvious to me what’s going on.
Leaving the fact that this figure also includes Hamas members, and therefore is useless at estimating if there is excessive collateral damage