Every country has borders, Americas are by far the weakest & easiest to exploit. No other country could survive with the number of illegal immigrants that the US has to deal with. Walk into any other country & say hi I'm here & I'd like to become a citizen here. You'll get a free ride back across but if you attempt it again chances are good that you'll be banned. It is a simple matter of $
America built a wall against immigrants in order to stop illegal immigration. However, there have been complaints by several people about the fencing idea and how it is not the most efficient way to deal with immigrants. There are no efficient ways for crossing for the citizens.