No, you do not have to buy the game and any pack you have again if you uninstall and reinstall the game. Doing that is the way to fix a problem with corrupted game files. I would advise that you rename your Documents>Electronic Arts>The Sims 4 folder to something else to ensure that you don't lose your saves. Or you could just copy the saves folder and put it out of the path of the game.
But more to your issue, what it sounds like is that the game has created a new user folder (The Sims 4) instead of using the one you had. So, are your looking in the right place? If you have ICloud (Mac) or OneDrive (Windows) cloud services active, that could be the cause for not seeing the files. The Sims 4 does not play well with the cloud so the best thing you can do is exclude it from your cloud service if you use one. It is best to keep all files, both game and user folder, together on your computer
Hope this helps.
