Because upstream LineageOS doesn't support microg out of the box. You can install it but it needs signature spoofing to pass Google's SafetyNet garbage. Bonus point for some roms that allow you to relock the bootloader after the install (iodéOS, CalyxOS).
Lineageos supports signature spoofing for microG these days! It did take them a long time to come around but they did in the end.