This is the goal of fewer people in power than you think, and keep in mind there is more border than the one between Mexico and the US. Most undocumented people have flown here and simply overstayed their visa. On the other hand, congress has been saber rattling about the southern border my entire life and has done little about it. It's useful as a wedge issue though, so from time to time, you'll see some spending a lot of money on walls and towers that just aren't very effective (and maybe callously separate some kids form their families).
On the other hand, undocumented people pay billions in taxes, work for cheap and make services (like dining, construction, landscaping, cosmetology, etc.) cheaper. The monied and political classes want to keep this status quo.