Why did Israel steal Palestinian land ?
Simply because Israel can. Israel follow the motto of might makes right. In 1947–1950 newly arriving European immigrant Zionists stole most of Palestine and ethnically cleansed 80% of the native Palestinian populace(at least 750 thousand).
In 1967 Israel took the rest of Palestine, and expelled another 350 thousand Palestinian native.
Here are some of the main ways in which Israel is stealing land from Palestinians after 1967: