Why? Because they need them for propaganda purposes.
“Hamas ‘bars injured leaving Gaza,'” from the BBC, December 28 (thanks to UsorThem):
Egypt says the Hamas militant group, which controls Gaza, is preventing hundreds of wounded Palestinians from leaving for treatment in Egypt.
Cairo says dozens of empty ambulances are at the Rafah crossing – the only one to Gaza which avoids Israel.
Hamas said it was drawing up lists of the injured but that it was difficult getting them to the border because of the ongoing Israeli air strikes.
Egypt has summoned Israel’s ambassador to demand an end to the bombardment.
Hamas officials say 271 Palestinians have been killed and 600 wounded since Israel began its aerial assault on the Gaza Strip on Saturday, but none of the injured have yet left via Rafah….