Joy-Con joysticks use a potentiometer to read the voltage at a wiper that slides across a strip of resistive material. That material wears down over time, or plastic and dust can dirty the sensors.
Stick drift is a huge problem with other Switch models. One survey found that 40% of Switch owners had problems with their Joy-Cons drifting, and things didn’t get any better with the Lite or OLED editions. After a bunch of lawsuits, Nintendo’s president even admitted it and apologized, setting up a free repair program for customers in some parts of the world.
Honest question; was it not possible to mod the original JoyCons to add hall-effect joysticks?
Yes, end users should not be responsible for having to do this - but if a cottage industry exists to repair/upgrade drifting joycons that would be awesome to see.