@Pantherina they could simply support phones with stock Android and there will probably be less work to do than supporting the broad range of devices that Lineage has.
They’re different projects with different goals. Graphene is more private & secure if that’s the thing that matters to you. Lineage supports far more devices
Graphene prioritizes security above everything else. This means that it does thinks like ship proprietary software if they think it will boost security. Lineage os is designed to be a Linux distribution for phones and is focused on providing a system that is all free software (excluding firmware). It doesn't kill off support for devices and is designed to be tweaked and doesn't support bootloader relocking out of the box.