Because it takes a lot of time and because the comments can get outdated. I also want this for all my code bases. But do I always do this myself? No, especially on green field projects. I will sometimes go back and annotate them later.
If someone didn't take the time to update the comments and the reviewers didn't point it out, then you've probably found the bug because someone was cowboying some shitty code.
Outdated comments are often way worse than no comments, because they can give you wrong ideas that aren't true anymore, and send you off in the wrong direction before you finally figure out the comment was wrong.
if (X) assert(false); // we never do X, ever, anywhere.
if (exact same X) { do_useful_stuff(); }
enum kinds { writers; readers; updaters; }