Comment by tombert
I mean, I think the answer to this is the very simple: they think it will lead to more money.
I'm sure someone in a board meeting saw something about GrapheneOS and LineageOS and Cyanogen and feels like if they de-open Android, some (or most) of those users will move to vanilla Android, and that will lead to profits.
I'm not saying that they're right about this; I think ultimately very few (if any) people actually know how to run businesses and it's all about giving an appearance of maximizing profitability, and as long as it leads to a potential short term stock boost then these executives get their huge bonuses and they can just blame the next guy when things break.
This isn't really theoretical; look at how Jack Welch took one of the most respected companies in the world, more or less integrated ponzinomics to temporarily bump the stock prices, and 20+ years later GE is kind of a joke and isn't even on the S&P500 anymore.
I don't have exact numbers, but I'm sure Graphene, Lineage, and all of the mods combined are much less than 1% of all android users; as well as these customers being less profitable than average as a marketing target.
Posting this from my lineage phone.