That's just Ubuntu Touch thing, but there are other distros out there. You shouldn't think about phones like Librem 5 or PinePhone the same way as you think about "Android phones" - there's nothing that ties them inherently to a single operating system. They're rather more like a PC where you can choose your distro by yourself.
However, if you're looking for "out-of-the-box GNU/Linux experience", Librem 5 comes with PureOS, which is essentially a FSF-approved Debian. See https://puri.sm/posts/what-is-mobile-pureos/
However, if you're looking for "out-of-the-box GNU/Linux experience", Librem 5 comes with PureOS, which is essentially a FSF-approved Debian. See https://puri.sm/posts/what-is-mobile-pureos/