Because it has worked to our advantage throughout our history. We settled the country with immigrants, and there's plenty of country left. Our aging population needs support from the younger, and if immigration is controlled we can get productive workers who will support the aged. It has now ceased to work to our advantage because it is now uncontrolled, and we now have in place a system that rewards those very potential immigrants most willing to flout our laws and least likely to be productive.
Generous is not unlimited. The flood we are now experiencing is raising not just resentment in the legal residents and citizens, but alarm. The critical mass was reached and surpassed long ago.