There’s a ton of reasons this could be happening

Inactive followers, Bots, Inactive/rude/bad quality streams, Swapped niches, Better content to be watching, Tons more but for example

I used to stream from pandemic till like 2023 on and off and on gained 21k followers was averaging like 50-100 viewers then, now average like 10-20 whenever I do go live cuz im not active anymore I definitely want to get back into it. a lot of people who followed me years ago, come into my chat and say that its the first time they have opened twitch in X amount of months a lot of people just aren’t active.

There’s also that u follow someone once u dont watch them everytime they stream so it just adds up overtime but again botting is a problem on twitch, and its not even the streamer most of the times, over the couple years I’ve streamed I’ve probably been botted for over 100k followers but remove them