Why do dead dry cell batteries leak, and . . .

. . . after all the years of making billions of these things, why haven’t the manufacturers solved the problem?

Dead batteries. A fact of life. Replace it. A dead battery that ruins your device is far more insidious.

Even the modern alkalines will leak. I seem to remember that EverReady used to claim that their battery was leak-proof but that was one of the most outrageous false claims in advertising history.

So what’s the SD?

It’s pretty hard to mass-produce an inexpensive metal can (that’s not hermetically sealed) that can forever contain a corrosive liquid. Yet another reason to use Lithium rechargeables- I’ve never heard of one leaking (although they do burst into flame now and then).