Why hasn't someone sued the Feds for not enforcing immigration laws in the USA?

My personal experience is that it is difficult to file suit against a governing body but this is such a core issue. Any legal beagles have a simple answer on this.
What would a private bloke have to do to sue over this kind of thang?

You CAN sue governments, to the extent they waive their "sovereign immunity," BUT you cannot generally sue goverments or their officials for exercising their discretion, rightly or not, in the course of conducting government business.

I might be wrong, but I thought you can't sue a "government". You can sue someone specific, but not the government.

They have. California has brought suit demanding relief from the unfunded mandate of caring for the massive numbers they provided services to in accordance with federal law.
Unfortunately, the U.S. Government has to agree to go to court when you want to sue them (us). They didn't agree, knowing it would come out against them.

same reason no one sued the FEDS when the Japanese were mistreated and abused! they are going to wait until people are abused and mistreated and then in the future receive a huge lump sum even if they are illegal!!