They make contact fine at the negative end. Why is the positive end different? The only thing I can think of is so it’s easy to identify the positive end.
Even apart from the labeling function, differentiated contacts work better. A spring against the flat side pushes the other end’s nipple into a socket. That’s easier than trying to work a battery in between two springs, and more reliable than a double socket.
Also in many applications, you’re putting multiple batteries in series. Getting a good contact between two large flat surfaces is harder than having a nipple make a good contact against a flat surface.
You may have noticed that the “negative end” is, in fact,* the whole surface of the battery except for *the nipple!
Although, in many small devices I put 2 batteries in, the contacts are identical with the two batteries placed in opposite directions to be in series - the negative is against the spring clip on one side, positive against the spring clip on the other. One has to look at the picture underneath them to get them the right way around.
I’ve never seen a device with that arrangement. In any battery compartment I’ve ever seen with a spring at one end, it’s always been at the negative end.
One right in front of me: My Garmin Oregon 550 GPS unit. Although, on inspection I note that it has a spring clip at each end. It’s still identical compartments to stick the batteries in, which you have to disambiguate by the illustration under the batteries.
Form loving function. Definitively for identification. How do you replace the batteries in your flashlight, if your flashlight doesn’t work because the batteries are dead?
Until the recent LED flashlights, the old incandescent ran fine if both batteries were backwards as long as the non-spring end was not recessed to only accept the nipple.
I think all of these answers miss the point. On older batteries, carbon zinc perhaps, the positive electrode inside is a carbon rod. The end cap has a little cup formed into the end that is crimped onto the end of the carbon rod to hold it. On the outside of the end cap, that cup is the nipple.
I think using the shape to distinguish the positive terminal evolved out of that manufacturing method.
The nipple is for sucking the juice out.
All carbon batteries - zinc/manganese and zinc/chloride (aka heavy duty) are made like this. Alkaline batteries are sort of the other way up - the central spike is the negative terminal, but to maintain compatibility they still use the same external format as the older carbon cells.
I don’t think the other points totally miss the point: most of the old battery designs did NOT have that crimp to hold the carbon rod. One of the few remaining examples is the 9V battery (which used to be made with layered cells).
Also, there are lots of batteries without nipples.