Hello and welcome to the forums.
There are many reasons.
A modern OS like Windows does not just sit there and do nothing. There are many background tasks running, including resizing of the page file, backups and restore points being created, updates are being downloaded. Also, many programs like your anti-virus are also running in the background, creating log files, updating etc.
But the one that will take up a lot of space, and likely is where most of your missing space is going, is the Windows Restore feature. I believe it is set to use up to 10% of your drive by default.