It happens because from normal use the potentiometer inside the joystick wears out. Usually you can buy joystick assemblies for a 1/10 of the price of a controller, so if you can solder its very affordable to repair them.
If you dont want to solder, you can extend their life by applying a bit of contact spray to the potmeters inside after disassembling the controller. But that can corrode other components, so be careful with it. Still beats just throwing them out.
No, this is the first time something like this happened to me. i dont even get it. In my country mostly young people speak english. Isnt the target demographic the elderly?
As far as I know its the graphite parts inside the potmeter that wears down.