Janitoring (and moderating) is a volunteer position that requires non-zero effort and occasionally exposes people to some bad stuff they don't want to be seeing.

Reasons for attrition go:

1. Janitor realizes they have to remove illegal content from 4chan, and they do not want to be seeing this content. This isn't a common reason, but I can imagine it might contribute to some other reasons. "I didn't realize what I'd need to do."

2. Janitor realizes this position requires non-zero effort and are not prepared to commit to a long-term period of effort for >zero compensation. "I've got better things to do."

3. Janitor feels that their effort is wasted or unappreciated. Especially true on larger boards that have unrelenting shitposting where you can delete shitposts and ban request people for an hour, leave for half an hour, and then come back to exactly the same stuff you've left. By nature of our positions, we can't talk about what we're doing or why to the posters and 4chan users like to blow things out of proportion. It kind of sucks to be doing something for what you think is a good cause, only to have the people you're trying to help turn around and call you names. "Why am I even doing this thankless repetitive task?"

4. People move on from using 4chan. Yes, I know "you're here forever" but believe it or not some people do eventually stop going to and posting on 4chan. A few years is enough to change a lot in a person's life, especially seeing the demographics that 4chan appeals to. If you're a janitor at 20, then you graduate from college and become a wageslave (or get kicked out of your parents' basement), you suddenly find yourself disinterested in what doesn't even qualify as "hot air." "I don't really care about doing this anymore."