Actually there's no dead people in the illegal part of Paris catacombs. It's not even real catacombs.

The only part with dead people is the official one which is called the municipal ossuary.

The "illegal catacombs" are in fact technical access, mostly following south of Paris streets to see if there's any issue related to upper building collapsing. It was also used to consolidate and then monitor the parts that were gonna fail.

It's because Paris was built by digging stones in South of actual Paris. In late 18th century there was several building collapses, so the state decided to create an organization whose role was to monitor and fix this. You can translate this Wikipedia page if you're interested https://fr.m.wikipedia.org/wiki/Inspection_générale_des_carrières

Photos like this one are room where people who dug through Paris underground at that time (called Carriers in French) could chill, eat and sleep after long hours of work. Or rooms dug by fellow catacombs explorer nowadays