Plain Linux on phones is still quite bad. It's not unusable like it was a few years ago, but it's still not good enough to gain any traction. Jolla is trying, desperately, and it's not working, even with the ever growing anti-American sentiments.

For Motorola to partner with one of the Linux phone projects, someone would have to invest significant resources in mainlining the drivers, replacing blobs with open source drivers where feasible, and maintaining that code when new upstream firmware and drivers make it downstream with patches and fixes. Looking at postmarketOS, you can see it takes years of community effort to port a device to the point of becoming useful. Once the software is done, the hardware is outdated enough that Motorola won't be making any money on sales any more.

In theory all of this would be a lot easier if Qualcomm, MediaTek, and the other SoC manufacturers would take the burden of mainlining drivers upon themselves the way Intel and AMD do. With the recent high-end Qualcomm chips, the company does seem to put in some effort, but these companies simply don't care about Linux support.

GrapheneOS is an Android fork so of course they're partnering with an Android company. They also don't have the capacity to maintain their own kernel + security patches + drivers, which is why they rely on upstream maintenance (from Google, historically) with their own Android-level improvements to remain secure.