[igneo676]

Mainly historical reasons:

Back in 2009 during the Cyanogenmod days, Google issued a C&D to the developers to keep them from distributing Google Apps alongside the main ROM. IMO it was less about the app distribution and more to force Cyanogemod to come to the table and work with Google to develop ground rules on how 3rd party ROMs would interact with Google more broadly. Cyanogemod (now LineageOS) basically agreed not to step on Google's toes. At the time it was not to distribute Google's Apps inside of the ROM. Now it's to not bypass OS level protections like Play Integrity (formerly Safety Net)

Their stance now can be found here: https://lineageos.org/PlayIntegrity/ . Note the part that says:

> Any action taken to bypass Play Integrity risks a backlash against all custom OSes, and could cause Google to block them entirely from the Play Store.

So long as the main players follow this advice, Google tends to also ignore smaller players that _are_ working around this via Magisk or other means. It's also possible that this simply becomes non-viable after some time.

It's also worth noting, Google has ways to allow third parties to certify their devices on https://www.google.com/android/uncertified/ . This doesn't grant fully Safety Net, but it's definitely another way Google is working with custom ROMs to ensure you have access to the Play Store