You can turn it off in following way: Admin panel → Left menu → "Settings" item → "Media" item → "Media settings" page → "Uploading files" heading → uncheck box "Organize my uploads into month- and year-based folders" → click "Save Changes" button.
When you uncheck this box, all future media uploads will be made directly to "wp-content/uploads" directory without automatically creating child directories for each year and month, incl. empty.
The reason for creating these empty directories is a small bug in WordPress code that prepares a directory where it can upload files via an API that is not always used as part of upload process. Aside from being an eyesore, these empty directories have no effect on site size, performance, or anything else.
What is the reason for this? By itself, WordPress does not just create these directories, they are only created during the process or attempt to upload files. Who does it? If the site users themselves do not do this, then some bots do it. For example, spam bots can automatically constantly make attempts to upload any files to the site through the corresponding CMS API. That's why CMS creates these empty directories for each year and month on the server on the 1st upload attempt in each new month of each year.