The idealistic answers that you mention here occur because people who pose topics such as "My work isn't being noticed and it's frustrating," create the impression that they are looking less for critique and moreso for reassurance and motivation to continue their work (which in itself is not a bad thing).
For example, here are two variants of a similar forum topic:
- Why is my art so crap? Should I even keep going?
- How do I improve the detail of my line art?
While both questions regard the notion of having 'bad' art, the first question is vague: it's emotively charged and non-specific, thus calling for more non-specific and emotionally cautious answers. Questions such as this indicate that the creator in question is distressed and unsure of things, which is something most people here go through.
On the other hand, the second question calls for specific answers and practical solutions. It differs from the first question, in that it seems less a product of distress but more so an inquiry for a tangible problem. The first question does not necessarily pin point areas for improvement, while the second is asking for a specific solution.
tl;dr - Critique does occur within these forums when the person asks for it. The positivity is there to keep everyone in high spirits--being an artist/author/creator is already a lot of suffering as it is.