First off - you do have a point. A lack of constructive criticism does mean that artists can stagnate. Surrounding yourself with all praise and no critiques means you will have a harder time seeing the flaws in your own work.
Secondly, though, I agree with @Shazzbaa - one of the reasons I rarely give big critiques (which need to be thoughtful and serious, not just "oh my god you suck at drawing faces practise anatomy") is that it takes a LOT of time. Not just in reading the work you're critiquing and writing out what you think, but in considering how to phrase that critique. There is an art to writing a proper critique that is more likely to be well-received and acted on by the person who is critiqued.
This can take hours, if not more, depending on the length of the work - because if I'm going to critique a thing, I'm going to want to have read all of it, so that I can put the work in some sort of context. I need those hours to draw comics.
Thirdly, not everyone here is looking for critiques. Some people here don't care about getting better at art, they're just doing it for fun.
And finally - I try to keep a positive tone even in my critiques. It's not about avoiding being critical so much as it is delivering that criticism in a friendly way. It is possible, and it is certainly a lot more likely that someone will receive that critique as constructive and helpful rather than hurtful. It hurts when someone comes in and tells you that this thing you love? This thing you've spent ages working on? Well, it sucks and you need to do better if you ever wanna get anywhere. And while there's a place for that kind of harsh wake-up call in criticism - some people do need the shake-up - I find that a lot of people who ask for criticism are already striving to improve, so they don't need the harshness. They just need an outsider's view on something they're already aware is imperfect.
TL; DR - it's possible to give helpful critiques and still be nice and friendly.
