And Mexican's can't build rafts? How many Cubans in makeshift rafts made it 90 miles along a narrow stretch of ocean to evade Coast Guard cutters with the latest technology and set foot on American soil? Build a wall and there is a LOT of coastline for them to use.
To borrow a line from the right wing book of Armaments when debating gun control- I would rather use that money to enforce the laws that are already on the books that to build a wall that will NOT stop the problem.
Right now, the number of illegals from Mexico is dropping, and the number of Asians and Eastern Europeans is rising. Are they getting here by flying to Mexico City and then coming in from the south? Assuming your wall stops all the Mexicans, they are about 50% of the illegals. So you've spend billions and have only solved half of your problem. What's next?