There's a lot of replies on here and I didn't read them all, so this probably has already been said in some form or another...
but here's my reasoning behind it.
-A lot of people ask for advice when they have like 3 updates out (or even less), there is literally not enough story to go off on to determine if they are good at story telling or not.
-If you do have a enough updates for me to see how you write....unless you're a really really good friend, I'm probably not going to read your story with enough focus and attention that is warranted for me to give you this advice, because to me it's really dick move to just say "You suck at story telling, go improve" without giving more concrete advice that helps them understand WHY I came to that conclusion and what are ways for them to actually improve. Like giving specifics (Here you don't develop the characters enough, here you have a plot hole, here it isn't clear why these people do these actions, etc etc- while also wanting to point out the aspects that are good and should be continued) I mean- become a better story-teller is a heck of a lot easier said than done, so unless I want to commit to this person and really help them improve that skill set, it just seems demotivating to say it.