There's probably just as many or more who travel by bus to and from Mexico every day. They are boxed in with each other for many more hours than on planes. You see them all the time on I35 going N and S, and on I10 going E and W. You really have to be careful around lunch and dinner time because they'll pull over onto the side of the highway and open the doors. A few dozen Mexicans will pile out and run across the grassy divide to the frontage road where there's a parked lunch wagon. Sometimes they'll pile out at convenience stores and you don't want to be anywhere near there at the time.
Oh my gosh, you are absolutely right. I had completely forgotten about the buses they use. We have a line that runs down I-25.