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)
It's not just the OS makers. They're also responding to the demand of companies and governments to control their users through them. They will not say "no".