Because you can't exactly sneak on/off of an international airplane. It should be pretty trivial for customs to funnel travelers through a checkpoint where they can search at the time of the actual border crossing.

Likewise we now have the technology to automatically surveil the border 24/7 (maybe modulo tunnels, though I wouldn't be surprised if we could detect those too?), so a 100-mile exception only ever becomes more and more dubious. We could've built the wall and lined it with a surveillance system for less than the cost of this Iran war (never mind all the m̶o̶n̶e̶y̶ ̶p̶r̶i̶n̶t̶i̶n̶g̶ stimulus this decade), shutting up the whole border debate once and for all, and ending justifications for internal surveillance for trafficking.