Some people have a need, a compulsion, to make insignificant things seem important. They have a need to talk about the things that are important to no one, often, not even to themselves. They do this because it gives them something to talk about. By talking they get to feel like they are conveying a particular perception of themselves, or they get to feel like they are a part of a relationship. Speaking pointlessly gets them attention (even if it is the negative kind) and it allows them to stimulate a reaction. It makes them feel less lonely.
Often children are annoying for pretty much the same reason: the need to comment on everything they can. Some never grow out of it. It is also contagious. People who are annoying often raise annoying children. The children never learn that pointless patter is unacceptable to many since they grew up around it. They grow up to be annoying adults and the cycle continues.
These people never learn what it is they are doing wrong, they wear their capacity for poor choices on their sleeve. It makes them easy to pick out at parties. When they get old they are considered “bores”. They shift from being merely annoying to becoming a full-on bore is largely about age and age-ism.
Even when they get called out on it, it makes no difference. You, the critic, becomes the problem, rather than their inability to keep their mouths shut and their banal ideas to themselves. They rarely get called out on it so if most people have no problem, then you must be the issue.
Most people have endured bores before, and have different skill levels/ tolerances when it comes to dealing with them.
What is a bore?
Is there a common definition or is it subjective? There are things that are of interest to very few, and there are things that are of interest to no one but the speaker. It could be that the person you consider boring has chosen an obscure topic, and is boring and uninterested party with it, more likely, though, they will speak of things that are of no value to anyone but themselves. This is the best point of consensus for the bore: could this have any value to anyone? People who think out lout are broing to everyone.
Why they are tolerated
They are tolerated because no one wants to offend over something as trivial as being bored by someone’s conversation. They are also tolerated by other bores who endure so that they can get a chance to spout their own worthless nonsense.