And ICBM’s made it impractical.
The pendulum has swung on that. We have decimated our air defense system (ADC, Nikes, etc) in favor of strategic missile defense oriented towards intercepting ballistic missiles.
That has left the low level missile attack envelope virtually undefended. The only real defense we have for that now is ANG groups with a few airplanes on alert at selected bases. Each airplane has about 4 AA missiles (AIM-9x/Aim-120) to contend with a barrage of cruise missiles.
At best we likely can put up about 30 airplanes on two coasts and the gulf. Even then, there are some areas that lack defensive radar coverage instead relying on ATC radars. Not so good if cruise missiles don't squawk an ident code.