For some the novelty of lemmy dropped pretty quickly. Most reddit users which make up a huge chunk of lemmy users would go days if not, weeks without commenting or posting. You kinda have to factor in that a lot of people are lemmy lurkers that will comment or post once they find something that interests them.
Also in my case I made several alts in the beginning (because I couldn't decide on an instance and I like to have backups for when an instance is down).
Other users probably did the same, which results in a bunch of inactive accounts.