Why don’t you build a free UI ontop of Mer (Sailfish OS)? Or resurrect Firefox OS? Or insert-name-here?

Because we want to promote a pure and unified stack, not have a separate mobile OS with proprietary bits or a completely different middleware stack. We want to support the community efforts of GNOME, KDE and UBPorts, and allow for any GNU+Linux to work out-of-the-box providing mainline improvements that work not just on mobile but across the device spectrum. The Librem 5 is a new approach to use a regular Linux system and adopt it to mobile use-cases instead of creating a completely new system. We do not create a walled garden, instead we tear down these walls, creating an open utopia. A fully standards-based freedom-oriented system, based on Debian and many other upstream projects, has never been done before–we will be the first to seriously attempt this.
You can also learn more about our position on GNOME and KDE further below in this FAQ.