They do have a quota and we do hold them to it.
But that is only in regard to LEGAL immigration. Those people have to go through hell to get here and to get their papers in order. Those who get here illegally don't have to do anything other than steal someone else's ID and then go steal another job from Americans.
Just set a number and anyone over the number gets sent back. If illegals are streaming over the border, then nobody in Mexico gets a visa until enough illegals are sent back to provide openings in the quota.