That's because you usually have many projects and your time is valuable for doing the actual editing - it's a waste to also do this job. But one of the General Editors of each drama can be assigned to do this, or they could take turns. It doesn't take that long. As soon as the subs are 100% this kind of comment stops, so that's a good time to delete them all.
Personally, as I said, I like having timed comments on all the time, so it irks me when I'm watching as a viewer.
That's why I do a first pass before watching the drama (okay, some of the comments spoil it for me, but I don't mind spoilers at all), so that I can watch it comfortably. And then another one when it's at 100%. If I can't be bothered to go through them all (in some dramas they are A LOT!) I just do a search with CTRL+ F putting certain keywords like "sub", "legendas" "%" and variations of F-words including acronyms.