Because it doesn't make sense.
Arab leaders told their people to leave the newly formed Israel while they 'dealt with the problem' expecting to wipe out the new country and return to their homes. When that didn't happen, rather than being allowed to settle where they had taken refuge, like most other refugees worldwide, they have been kept in camps both within the Palestinian territories and in neighboring countries.
The late 40s and 50s were a period of huge global migration, but the only refugees not allowed to resettle are people from, the Palestinian territories. Can you explain why that is?