The reason why auto-install doesn't work for major version upgrades is because Google Apps are version-specific, and LineageOS isn't allowed to ship Google Apps without paying for certification by Google (and, in fact, disregarding this _will_ result in cease-and-desist letters).

This means that ~95% of users would break their installation by pressing the "auto-update" button because they didn't update their GApps accordingly.

Also, nowadays there is the problem of requiring a certain version of the built-in firmware as well, and in case there are any incompatibilities, a major version upgrade is the perfect time to require updating that as well.