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.
Consumers didn't pick up Windows Phone or HarmonyOS enough to matter either. Access to the two common app stores is crucial for user adoption even when the UI is good.