There is another reason: It’s the least bad option for some people.
The business model of Google dictates, that you are the product if you use vanilla Android or whatever it is Samsung slaps on top of it. If you go with a custom image, such as LineageOS, you do get more freedom, but there are also compromises. If you choose to not install GAPPS, you get plenty of privacy too, but the compromises are getting pretty severe at that point. Realistically, using a phone like that isn’t for everyone.
If it was still 2010, and the world around me didn’t require certain applications, I would probably be going full on FOSS with my phone. Sadly, we don’t live in that world any more.