You need to enforce it because illegal immigration is harmful in and of itself, even if the immigrants aren’t criminals or national security threats. Why do we enforce speed limits even when the person doesn’t cause a serious accident? Because the point of the law is to create a deterrent effect that compels people to follow a certain process.

Obama had an easier time deporting people because, at the time, most people in his party accepted the view that illegal immigration is harmful even without some other crime: https://www.foxnews.com/media/2010-obama-clip-goes-viral-whe.... Back then, even most Democrats embraced requiring immigrant to assimilate. If you think assimilation is important, then it naturally follows that we have to control the number of immigrants at a level where America changes them before they change America. Today, many of them reject assimilation in favor of multi-culturalism. If you embrace multi-culturalism, it’s hard to justify any limit on the number of immigrants. And at that point, illegal immigration just becomes a technicality.