There have been many laws passed by Congress addressing immigration. It is against law to cross the border without authorization. This particular case exists as a result of not enforcing those laws. Pretty simple.
Despite past congresses passing laws, the system is not good. A major contributor to the problem of people immigrating illegally is that our legal immigration system is a total mess. This is why there have been a number of efforts at reforms over the past few decades. But none of them have worked, leaving us in this situation where presidents from both parties do all this illegal stuff by executive action.