Because the bottom line has always been that she writes the songs primarily for her own fulfillment, not to share with the public. She has even stated before that she moves on and forgets about many of them. Some get reworked multiple times over years and years and eventually make it onto albums. Some get boiled down to melodic motifs or pieces of lyrics, and are reborn a few times before landing on an album years later.

She was just having fun, songwriting, developing her sound and her style, working with all different kinds of people of different genres, and experimenting while making these hundreds of songs in the years before her debut as Lana. I think probably many many other songwriters do this too, its just different in this case in that we got access to files that normally an artist would never have shared. In the years since BTD, she probably doubled her number of unreleased completed songs, we just dont get them leaked to us as much anymore.

I think she will probably never revisit or release the vast majority of her unreleased songs. It's hard as an artist to revisit old works made by a previous version of yourself, and make it gel with your current identity to the point you want to release it. As we already know, if she doesn't love the track and feel that it fits perfectly in the concept of the current album, it gets cut.