Chrome is unfortunately limited here by the security of the OS. No popular desktop OSes have application isolation: all apps have the same permissions. Any app can write to any other apps' storage.
This means that if Chrome makes sideloading too difficult, developers will just tell users to run their native code which will hack into Chrome, making even understanding what extensions users have or uninstalling them impossible. Sideloading on desktop OSes has to be hard enough to discourage most users but easy enough that developers like adblock don't start looking for an even bigger hammer.
This is what I meant by delicate balance of incentives.
More info here: https://news.ycombinator.com/item?id=4954915