If there was an in-game parser I feel like a lot of people would probably improve because of it.
I know that until pretty recently I had no idea my damage was so poor and it wasn't until my friends who play on PC and can use ACT told me about it and I actually tried to improve.
Yeah there are people who won't try either way but there might also be a lot of other people who simply don't know how they should improve, or even if they need to improve, because the game doesn't have a parser.