The community of Framework users is too small currently for the likes of LG or Samsung to care. And custom displays are extremely expensive to have made. I think the Framework-13 just uses a display that was already available. Even with the Framework-16, the display was based on an existing screen. Framework mentions having it modified to improve the specs.
There are a few alternate displays that you can use in the Framework-13. It’s just that no one has found one that is really better than Framework’s choice. The screen ratio isn’t as popular, so the choices are limited. For the Framework-16, I’m sure once people get into it, they’ll find alternate displays there too.