There are several reasons.
The first
Free plans for online storage such as Google Drive, Dropbox, MediaFire, etc. are too small to use and are inconvenient to use. Not everyone here has a paid plan.
File uploaders such as Katfile, Rapidgator, and Uploaded will give you 2TB to 5TB of space even with the free plan.
The second
In the case of major services such as Google Drive, Dropbox, etc, if a usage violation is reported, not only the link but also the account will be deleted.
With file uploaders, it's often just a link.
The third
People who upload many files frequently also need devices such as NAS. Some of the file uploaders provide affiliates so you can use them to cover the cost of your device.
Those who upload for the above reasons often select the file uploader.