I have 0.0 problem with normal people coming here, trying their level best to earn an honest day's bread, perhaps even get wealthy. Even applying for citizenship. But there are ways to do it, not slip across the border.
What we can conveniently forget is the one guy getting into the country with not so noble intentions, ending up here playing the system to get his buddies in. Think of teh little grains of rice on a checker board. 1 grain, 2 grains, 4 grains 8 grains ... you get the picture. These are the guys that are part of major crime syndicates. They cost our economy a lot, also you and me. These syndicates defraud local businesses, target our business partners abroad* from here, end up selling drugs to our kids and like ... it goes on and on. If think of crime syndicates, we tend to think of something mafia-like. Africa has it's own crowd, anthign from drugs, cyber crime to human trafficking. They quickly negate any benefit the bulk of the other people arriving on our shores/airports can bring not only to our country but also, but to themselves. A lot of this badness goes unchecked.
I know for a fact that not all the costs of deportation is only for our authorities, the US has been funding quite a bit in extraditions. (Watch this space)