This seems like a waste of material. It might make sense for the factory making process though. Also, after all those years of internet, the "don't blindly believe everything you see" rule should be always applied.
As a kid I liked to open up batteries to see what’s inside and lick them to feel the shock and connect a bunch of them together and things like that. So I guess I have been aware of this from a very young age. If you open up the smaller canisters, there is a black goo inside which is the electrolyte.
You can get a surprising amount of power from batteries. You can also combine AA batteries for example to get a 9V battery with more available current. I started a truck with the battery from my Makita drill one time
Battery voltages come directly from the chemistry of the process. A carbon-zinc (old "heavy duty") battery is 1.5v new. An alkaline battery is a bit above 1.5 volts new and slowly drops off with use. A Nicad battery is about 1.2 volts until it is near fully discharged. A lithium-ion battery is about 3.2 or 3.6 volts depending on type.
If you want a higher voltage, you have to connect several cells in series. So 9V batteries have been ~6 cells in series since they were introduced, more or less. If you open an older laptop battery's housing, you'll often find several cylindrical lithium-ion cells. (Newer ones that are made to be very thin probably aren't built that way.)
this site above has pics of laptop batteries, but they're too big to post here.
Lately I have noticed the small footprint A23 12v battery. They are smaller diameter than a AA and half the length. Yet they have 12V. A23 batteries are constructed of 8 individual LR932 alkaline button cells enclosed in a wrapper.
Now I see there are many similar sizes I have never seen before: