Because he doesn’t care, and it would take effort. Same reason he did not “build the wall”.
While it takes effort, Congress could do it in a single, actual, bill that really enforces border security. Here's what they should include in it.
* Ends the possibility
ANYONE who is in the US illegally becoming a citizen or having a path to become a legal resident.
* Authorizes all state and local law enforcement to assist in immigration enforcement.
* Bans all "sanctuary" locations on penalty of law as aiding and abetting a criminal activity.
* Requires anyone arrested for illegal entry or being illegally in the US be held without bail until they are tried because they are flight risks.
* Bans persons illegally in the US from obtaining any sort of official government identification, to include driver's licenses.
* Bans persons illegally in the US from entering any sort of legal contract as they are doing so while acting as a criminal. This includes buying anything on loan, renting an apartment, etc.
* Bans persons illegally in the US from making wire transfers of money within or out of the US.
* Puts employers caught using illegal immigrant workers who were not charged criminally or civilly for that on an automatic 5-year probation period where if caught again they will be charged and tried criminally for hiring illegals.
* Bans persons illegally in the US from registering a motor vehicle.
That would pretty much mean that the vast majority of people illegally in the US would self-deport. They can't get a job, can't get ID, can't get a place to live, can't get transportation, and when caught they're going to jail until they're deported.