Also because when someone points out how 'irritating' it is, you don't necessarily get a very good feeling about how empathetic they might be! A lot of chattier people might be fine, but people get defensive when called irritating, and so are more likely to conclude unpleasant things about the OP (which might be unjustified!). But I don't think it means that most shy people are going around being choosy about whether someone is worthy of talking to.
There might also be a general difference of opinion on just how much social interaction needs to take place - all this 'grunt work' etc that people feel has to happen. Sometimes, people don't actually mind not talking, or not having lots of conversation going on that everyone is involved in. Yes, there can be awkward moments, and I think we're all grateful for people to make an effort then - and as a very shy person, sometimes I do really force myself to do it, however stressful it is, as the awkwardness also bothers me - but quite often, people feel that there should be conversation that everyone is involved in, even in situations where actually some people are quite happy just listening or just appreciating the atmosphere or the food or music or whatever.
Yes, shyness stopped me in my career. I have spend years getting over it - and I've learned to hide it to a large extent, and to force myself to do a lot of things that I didn't used to be able to. It doesn't mean that it has entirely gone away - in fact, by forcing myself to do things and hiding the shyness, I've developed panic attacks. I can go for so long looking normal and outgoing but feeling awful inside, and then suddenly something tips the balance, and it all comes out in a panic attack, and everyone is even more surprised because they weren't expecting it. And it draws more attention to me in a way that I hate. So don't assume a shy person isn't working hugely hard on it, and struggling a lot, even if sometimes they appear OK. It might be light years ahead of where they have come from.
Plus it takes huge amounts of practice, with the tiniest little steps. And when you are terrified of practising, of the people you have to meet, of the situations where you are almost always going to be out of your depth, it makes it hard. You don't want to go, clearly your relatives find you irritating, and feel that you shouldn't be there unless you are contributing more, so it can be hard to get much better at things!