Legalization of all 11 million would set off a population flow south to north like the world has not seen since the partition of India and could well include the violence that characterized it on the subcontinent.
Rubio has said that any legalization can only occur after the border is much more secure than it is today, for precisely the reason you point out. (Still not sure I favor legalization, but I concur that any such proposals are complete non-starters until it is much harder to enter the USA illegally than it is today.)