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)
If anything, it's the playstore and appstore which are side channels.
We could also imagine a mechanism to provide an update URL in the app metadata. The OS could query this URL periodically to check for updates.
So it's still a direct install, it's just that direct install support is limited on phones.