Why didn't I think of this 40 years ago?
Before 1940 American cars had a handbrake lever that was easy to manipulate with one hand, making it a suitable substitute for the footbrake when the hydraulics failed. Euro cars continued with hand levers, which became even handier when mounted between bucket seats. After 1980 Americans finally relearned the correct form.

In the '40s and early '50s American cars had various types of under-dash pull levers, which were unusable for emergency braking. From the late '50s through the '70s, most had a pedal with a release lever. This was usable exactly once, unless you were quick enough to reach down and hold the release lever up.

There would have been a simple way to restore the function. Screw a clasp-type gate latch under the dash, positioned to grab the release rod when you pull it up. The release lever then remains 'open' during driving, so you could use the parking brake pedal easily for hydraulic failure or fancy tricks. When you want to park, just hit the release lever downward out of the clasp, and apply the pedal.

Why didn't I think of this in the '70s when I owned some American cars with bad hydraulics? Would have prevented a lot of close calls. Too late now.

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Later, a much more practical why didn't. When running laundry, I normally use the sofa in the living room as a sort of clothesline. Pull out a sheet early so it won't tangle up the rest of the stuff in the dryer, spread out the sheet across the sofa. Airflow on both sides. This morning I was drying a flannel contour sheet this way, and decided for some reason to pull the corners across the top of the couch. Better stretching. After it had dried, I realized: The same sofa can help to FOLD the bloody stupid contour sheet. Leave the first two corners on the back, bring up the other two, pull them across the same points. Presto! The otherwise impossible first step is done. The sheet can then be held straight and folded. Sort of. It's still sloppy compared to "proper" methods, but at least it's better than my usual total crumple.

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Addendum: Apparently Cadillac had a "proper" method in the '60s and '70s. The ratchet would lock when the shift lever was in P or N, but stayed released when in DLR. So it was usable when the hydraulics failed, or usable for skid-turns by Bob Bondurant High Security Chauffeurs.
  ¶ 11:54 AM