Because the entry points are closed.
https://www.law.cornell.edu/uscode/text/8/1158
Any alien who is physically present in the United States or who arrives in the United States (whether or not at a designated port of arrival and including an alien who is brought to the United States after having been interdicted in international or United States waters), irrespective of such alien’s status, may apply for asylum in accordance with this section or, where applicable, section 1225(b) of this title.
They are actually following the legal process
The code link is cut off for me