Bull...
Airlines are required to return a traveler who seeks admission into the U.S. that Customs and Border Protection refuse.
The reason non essential is permitted via air vs land is based on the premise majority are present in the U.S. for a short duration. Car rental becomes expensive. Does it happen? Sure.
Though the crux is on land entry:
Via land, Australians, Asians, French, Germans, Mexicans (whole other story), and mainly Canadians via U.S. Canadian Border that drive their own vehicles fall under B2 or WT status as tourists. That permits their time in an RV, etc up to an <initial> 6 months (B2) and 3 months (WT).
Whether one feels that's a crock or not... speak with the airline lobbyists to start.
I'm not a fan of it though closing land borders has certainly reduced the quantity of "tourists" remaining longer than a week or two.
Side note: Entering the U.S. has Jack squat to do with a U.S. as a disabled Vet or an escaped convict returning. A U.S. Citizen will not be denied entry into their home country - U.S. <period>.
The convict will be arrested, the Vet welcomed home as many of the CBP Officers are vets themselves.
Another article from the U.S. side:
www.traveldailymedia.com