That would be because the Arab states collectively massacred or purged Jews from their lands around the time of the 1948 establishment of Israel. Unlike the unique position of Palestinian refugees, no right of return or requirement for compensation is recognized for those Jews or their descendants, not that they'd generally want it. Also, Arab Israeli citizens and even Palestinians living in the occupied West Bank have significantly more rights and better living conditions than Palestinians hosted in Arab refugee camps. Many other Arab nations prefer to keep their Palestinian refugees herded into ghettos, and kept as pawns in their ongoing war by proxy with Israel. I'm not much of an Israel apologist, but I suggest you do a bit more research into the issues and history of the region. I think that you'll find that Israel's treatment of its non-Jewish population is generally poor and sometimes unforgivable, but far less evil than their neighbors' treatment of Jews or even Palestinians.
There must be more to life than having everything. -- Maurice Sendak