Honestly I believe at this point it's partially inertia and brand recognition. In the past, when Linux support was more hit-or-miss, and the likes of Dell and Lenovo had no linux support, I think it made more sense.

More charitably, they do some work on custom firmware, and according to themselves, "work with" Clevo on making hardware decisions. What exactly that means seems vague to me, but again, charitably, System76+Clevo are at least trying to avoid problematic hardware, while Dell and Lenovo ... don't so much?