It’s the same reason he has ignored the economy or illegal immigration or foreign policy or the national debt. All of these things require leadership as well as actually doing work - two qualities that are not conducive to someone who views the Presidency as a lottery prize.
The reason there is a welcome mat at the southern boarder is plain and simple. Obama wanted a "comprehensive" immigration bill passed that included amnesty. The congress refused to do so, requiring the border be secure first and foremost - meaning no comprehensive bill.
Obama's mode of operation is to create a complete disaster down there, flooding towns with illegals until they go begging the federal government to "do something!" Obama will then blame congress for not sending him his "comprehensive" immigration bill (which includes amnesty and increases in yearly immigration quotas).