People give unsolicited advice all the time in person and it's rarely as annoying as when the same thing happens online.
Sometimes (often, if I'm being honest), I will post about something here, and the post garners suggestions from people that I immediately find irritating. But if I step back and think about it, the suggestions are things that wouldn't bother me if a friend said them to me in real life
I've been pondering lately about why that is, and I think I have some answers.
1. Parasociality: Social media creates an asymmetry in person-to-person interactions that shows up as parasocial relationships. The result is that people might feel like they're giving advice to a friend, but to the recipient, it's like getting advice from a stranger. I think it's usually easier to accept advice from a friend because there's a sense that they know you and understand your situation.
2. Communication: Online communications are stripped of all the non-verbal cues that help lubricate person-to-person interactions. If your real-life friend makes a suggestion that you don't care for, you might sigh heavily or roll your eyes -- behaviors that communicate that you aren't receptive to the idea. I think those kinds of cues can carry more weight than a verbal explanation. Online, all a person gets is a verbal explanation, and those lend themselves to parsing and debate. And no one wants to debate something like that, especially if they are a stranger (see point 1)
3. Volume: If you're with a group of friends and complain about something, one of them might chime in with some advice, but everyone else is privy to that interaction. If you reject it, your other friends will know not to keep suggesting the same thing. If you accept it, everyone else hears that and you can move on to a new topic. Online, people don't always see and often don't bother to read other replies, so you end up with several people suggesting the same thing, even after you've already told someone else it won't work.
In short, social media sucks for socializing!