Aside from the people that abuse the service to host files (using free accounts or using accounts with sniped nitro) there are a lot of bots that do image/gif generation that upload directly as attachments. You could see people using multiple commands, while generating multiple gifs, until they get one they want to save. Theres also servers like Midjourney. I imagine a lot of what Discord stores is dead files that will never be accessed after the first time.

Another note is that a lot of the meme gifs that are used come from the Tenor and Giphy integration which are only temporarily hosted on Discord's media proxy unless a user downloads it and reuploads it.