At some point your thumbs wouldn't activate the pads so you had to use your thumbnails and then it was just a matter of time before the tester strip quit working.
Technology Connections did a video about those very batteries.
It's interesting that he speaks of them as something very old, but such batteries were still widely available in Europe not too long ago. I have a pack of Duracell PowerCheck AAA batteries made in Belgium and labeled good until 03/2029, which suggests a manufacturing date of 2019.