Because immigrants moving to a new country in search of opportunity and a better life is a bad thing and should definitely be deterred.

The social safety net is not he opportunity that I want immigrants to be looking for and I say that as a immigrant myself. If I had come to the US and ended up unemployed soon after my arrival, the US shouldn't have had to pay for me. If I don't like that, I can always go back where I came from. Of course it's easy for me to say since I would be going back to a wealthy European country with comfortable safety net and to middle class parents.