Because if 37 changes are done to the same 15 line function over time, the amount of comment material will dwarf the function. And most of it will pertain to historic versions of the function which are not what actually appears below the comment; a comment made 13 revisions ago makes sense for the 13-revision-old version of the function.
You _could_ tag every line of code with
but if you did it would become unreadable and wouldn't be kept up to date.