The problem is that most normal people (HN is not normal - mostly for the better) don't even understand what sideloading is - let alone actually care.
How can we fix this?
(aside from making people care - apathy enables so many political problems in the current age, but it's such a huge problem that this definitely isn't going to be the impetus to fix it)
The problem is - linux (outside on server land and maybe SteamOS) is everything but (regular) user friendly.
When people buy a new phone the expect a smooth experience without any major inconveniences and uniform UI. And apps. Lots of apps. Full of features and mature UI. Linux mostly have none of it.