Why are almost all packages marked as manually installed?

Seems my Google powers were weakened: this is a known bug (concentrate on comments from Colin Watson) and it's hanging around for some time with no fix in sight:

Quoting Colin Watson:
[...] Leaving all packages as manually installed is unsightly but the consequences of this bug are generally limited to not managing to be clever enough to remove packages that aren't needed any more. A flaw in the reverse direction would be much worse, because it could cause packages to be removed when they are still needed. Given that there does not seem to be a clearly correct fix available, it's better to air [sic] on the side of caution. [...]