In the 40's brakes were undergoing many changes. “juice brakes” became popular.
To prevent air from gerting in past the wheel cylinder cups they found putting a small check valve in the MC outlet kept slight pressure in the lines, keeping air out.

That was SOP until disc brakes became popular in the 60's.
At that time some MC's were dual outlets - both outlets had the check valves. Instructions that came with it said to remove the check valve if that outlet was to operate disk brakes, leave it in if it was for drum brakes.

In the 60's most everyone could read.

But sure enough, complaints started, lawsuits were started - people left the check valves in and the residual pressure of about 3 lbs was enough to cause the disk brakes to drag.
So - the answer - mfgr's and rebuilders left the check valves out!

At least then the complaints mostly went away - but the complaint EXACTLY like the OP's became popular. --- Air got in when parked.

Carmakers addressed the problem - they moved the check valve into part of the combination valve. They set it to about 1/2 lb of residual pressure.
Then the wheel cylinder's cup's feather edges stayed expanded keeping air out, and the square O rings in the calipers stayed rocked, sealing them against air intrusion.


WHEN YOU DON'T KNOW AND SAY STUPID THINGS DOES IT MAKE YOU FEEL GOOD THAT EVERYONE KNOWS?

 

Drum brake wheel cylinders only seal completely when there is pressure behind them. Some larger vehicles run 10# or more residual (drum) brake pressure.
If that pressure were not there, releasing the brakes would cause air to be sucked in around the seal as the pressure changed to vacuum. Drum brake shoes are retracted by springs, so the residual pressure isn't a big deal.
Discs are another matter, they don't typically use a metered check valve unless the master cylinder is mounted very low in relation to the caliper, but still maintain a fraction of a pound back pressure. If you pull a caliper and remove the pads, you'll see the pistons slowly extend.