It's not a matter of whether or not they're qualified, it's whether or not it's wise to trust strangers on the Internet.
In addition to curiousdannii's good answer, there are some general reasons for being skeptical of being able to trust the advice of people in a discussion forum, or a site like this:
- The web as a means of communication is far lacking compared to personal interaction. The ability to evaluate the sincerity and trustworthiness of an individual is hindered when non-verbal communication, such as eye contact, body language, voice inflection, and others are removed. It becomes harder to tell if someone is putting you on simply because those cues are missing.
- In addition to honesty, it's easier to evaluate someone's knowledge in person or on the phone. On this site, you ask one question, get an answer to it, and that's it. In person, you can ask follow-up questions. Admittedly, that's an option for a discussion forum, so the argument isn't as strong there, but it still holds water. Even in a discussion forum, conversation is different than in person or on the phone. Again, the nonverbal clues that help us to evaluate sincerity also help us to evaluate competency to a better degree than is possible online.
- I can't find any right now, but I know there have been studies to show that people are more likely to lie online, where they are semi-anonymous. Not everyone is fool enough to use their real name in sites like this.
The bottom line is that with a legitimate understanding that there are inherent flaws in online communication compared to face-to-face interaction, it comes down to a simple "you really don't know who you're dealing with". That's fine if you want reviews of cameras, or even sports cars. Probably OK for ideas for recipes, crafts, etc. I mean, you make a dessert that tastes nasty, so what? Lesson learned.
However, this site deals with Christianity, issues that we Christians believe have eternal consequences. We're not talking a junker car that has the transmission go out after 1000 miles, we're talking an eternity of unimaginable torture. (At least that's what many of us believe.)
The amount of risk you're willing to take is directly affected by the consequences of making the wrong choice.
- If you told me to flip a coin and if it lands on tails, I have to suck on a lemon, but if it lands on heads, I get a nice juicy steak, I'll take that risk.
- Tell me that if I win the coin toss and I get a million dollars, but if I lose, my child dies, and I'm going to tell you to buzz off, no way.
So the risk here in getting bad advice is high. From my perspective, an eternity in Hell means that I'm going to be darned careful on who I seek advice from. Why would I trust someone who has no personal connection to me, who has never met me, may be an idiot or a liar, and I can't even tell if they can look me in the eye when they give the advice? No thanks.
Granted, not all personal advice here is a matter of doctrine to the level of eternal security, but some of them are pretty steep in the earthly potential risks. I've seen people asking for marriage advice, suicidal posts, and others that are just too important to trust to faceless strangers. Those people need to be sure they can trust the person they're talking to.
Also, specific to OUR site, there are a lot of people with differing opinions. How are they supposed to know which is right? It's a dilemma discussed in Another reason this is not a Christian site When you go seeking advice on the internet, it's really easy for smart people and bozos alike to pipe up and give advice at varying levels of value, and again, there's that whole lack of visual cues to help you know which advice or at least which person to trust.