Because the Jewish civilians didn’t have another option: either they defeated them… or they were murdered or expelled. From the military point of view, the Arabs were defeated because of their lack of coordination. It is not the same to fight for survival, as the Jews did, than to attack others based on “abstract” objectives led by corrupt leaders. The Jews were poorly prepared for the 1948 War. The Hagana had 6 divisions (Golani, Givati, Carmeli, Alexandroni, Kiriati and Etzioni) and another 3 divisions of the Palmach (Harel, Yftach and Ha-Negev). During the war, another 3 divisions and a small air force (Force 101) were formed. There were 6,373 Israeli casualties in the war, 4,000 soldiers and 2,373 civilians. They fought 20,000 Egyptians, 18,000 Iraqis, 5,000 Syrians, 13,000 Jordanians and more…
The exact number of Arab deaths in 1948 is unknown: supposedly 2,000 Egyptians, 1,000 Syrians, 1,000 Jordanians, 500 Iraqis, 500 Lebanese and 3,000 more Arabs from Palestine… in total: around 8,000 people. Aref El-Aref, who also included the deaths provoked by discussions among Palestinian Arab clans, rounded the deaths to 17,000 (13,000 of them were Arabs from Palestine). The different Arab-Palestinian clans settled the scores among each other because during the direct war with Israel 3,000 of them died. The Arab countries signed armistice agreements with Israel in 1949, starting with Egypt (Feb/24/1949), then Lebanon (March/23/1949), Jordan (Apr/3/1949) and Syria (Jul/20/1949). Iraq was the only country that did not sign. The cease-fire lines were military agreements and not recognized borders. However, in Lebanon’s case, the cease-fire line was established in the international border of 1923 and with Egypt, the cease-fire line went through the 1906 border (except through the Egyptian occupation in Gaza Strip).