"Demonetization" is just what YouTube's promises to advertisers look like when they affect video creators.
Money on YouTube flows from advertisers. The revenue from charging advertisers to show ads is split between YouTube/Google and the video creator. If your video is not shown with ads, then there is no revenue to split.
YouTube gives advertisers a very small control over what videos their ads are shown on. They have a few different classifications of videos, and advertisers can choose which ones they want to be seen with. Advertisers are paying for the service of YouTube putting their ads on videos — but only the videos that YouTube thinks the advertiser does want to be seen with.
If your video is fully "demonetized", that means YouTube has decided that no advertisers want to be seen with it; or that they are not willing to take revenue from showing ads on that video. But they're still hosting it, making it available to viewers.
Video creators' revenue is a share of the ad income from YouTube showing the video (and accompanying ads). A "demonetized" video is one that doesn't show any ads — so there is no revenue to split. It's not that YouTube is taking all the revenue and leaving none to the video creator. They're not making any, because they don't think the advertisers would be okay with being charged to be seen alongside that video.
However, the creator of a "demonetized" video is still receiving value from YouTube. It is not free to host that video — especially if it is popular. Network bandwidth, data storage, and transcoding of video for viewers' browsers are not free; YouTube covers the cost of these. YouTube is willing to host a lot of videos that they make zero money from, at their expense, rather than censoring those videos by taking them down.