The problem is it's very easy to make entering the country hard for legal immigrants and tourists - you just make tons of strict rules, tell border control to enforce them vigorously and voila! But it's much harder to crack down on illegal immigration, since by definition they won't follow laws and rules.

You'll need to spend a lot of money and effort on finding them and getting them out, on enforcing eligibility checks at workplaces, etc. And face very strong political opposition - since it's one thing just not to let the person in and very different thing to uproot a person who already has been in for a while, got some local contacts, established themselves, etc.

Now add people that oppose virtually any immigration enforcement for political reasons, and you've got the situation US is now in - there are about 10M illegal immigrants, about 1M have final deportation orders (which mean the state ordered them to leave now, all process, appeals, etc. have been exhausted) and ICE has in custody about 12K (yes, 12K out of 1M), where are the rest - nobody knows and nobody will know.