Lightroom Classic 13.4 Windows - Develop Presets Missing

"Even if i try to import them again, i get a message saying that it's already installed."

This very much sounds like the longstanding bug Johan mentioned -- if you import a preset that refers to a profile that's not currently imported, LR will import the preset but it won't be displayed in the Presets panel.

"Just delete all the presets, and imported them from an old backup i had. Odd, but this way they all show up."

Presets and profiles are both stored in .xmp files in the same folder(s).  So most likely what happened is that when you imported from the old backup, you imported both the presets and the profiles this time.

And have you entered the manage presets to turn them on?

Do these presets apply a profile? Then they won't show if that profile is not yet installed. And have you checked the option to show incompatible presets? If not, then a preset that applies a specific camera profile will not show if you have selected an image from another camera.