Probably because they know FOSS is not at all a threat to their business. I love the idea that if I wanted to I could put some other OS... But Android is very reliable and Graphene OS might not support some feature or other, which I might not notice till I actually need it, so I'm not gonna risk the most expensive thing I own with tinkerer tech.
Companies really overestimate end users tolerance for tinkering.
> I'm not gonna risk the most expensive thing I own with tinkerer tech
Different strokes I guess. Until I've put the vendor in a box, I don't consider it "my" phone, and I'm not gonna pay more than $300 for a phone that serves some other master. Yesteryear's refurb pixels are doing just fine.
Companies really overestimate end users tolerance for tinkering.