Of course there are tunnels, so you find them and blow them up. Doesn't mean we shouldn't build a barrier, it requires a lot of time and money to build a tunnel which can then be destroyed as soon as it's identified. Decent surveillance will eventually find traces of tunnel activity, or a captured illegal will spill the beans if he/she used a tunnel to get in. Or, we can use it to grab illegals coming through and maybe get some really bad guys, like drug dealers, gang bangers, and terrorists.
You don't seem to get it, the wall is not and never was meant to be the be all and end all for stopping illegal border crossings. What it does it drastically reduce the numbers of crossings, as proven in places like Yuma, San Diego, and El Paso. In combination with other measures, we can really cut down on the numbers of illegals coming in without going through a checkpoint.