I think the reason is because LineageOS is listed as an approved Google ROM which passes the safetynet tests that google added to block custom roms. Some apps u

I think the reason is because LineageOS is listed as an approved Google ROM which passes the safetynet tests that google added to block custom roms. Some apps use safetynet to block users who have higher levels of access to their system. Allowing microg to work would put the google tick of approval at risk.