That isn’t entirely true. There were also hardtop sedans back in the heyday of American iron.
I shudder to think of how flimsy a hardtop sedan would be using unibody construction...
The doors would jam on “suicide door” Lincoln Continental hardtop sedans when jacked to change a flat. Pretty flexible.
Thought sedans were hardtops. Did you mean convertibles (retractable hardtops)? I have one, a ‘57 Ford Skyliner.