Raiyia OP, those of us who might tell our friends that we went somewhere really lovely for lunch have very much kept to the sentiment of the thread by repeatedly answering your But Why? question.
We've patiently explained the many reasons why (some use it as a diary/scrapbook; most are happy to see what friends are doing; some use it for ideas; people use it to keep in touch with friends with similar interests such as nice food, etc, etc)
The way this response is written is actually a great example of the strength of the ego involved. Note the almost royal use of "we" and the condescending description of "patiently explaining" (as if the ordinary person should be grateful for the time spent).
The majority thankfully don't brag on FB, they can tell the difference between describing things that happened to them in a way that's interesting and not boastful, sadly the braggers cannot. On and on they drone, blissfully unaware that they are making tits of themselves. Most of my friends on FB don't actually post that often, maybe between once and 3 times a week. I can't actually think why someone wouldn't be embarrassed if they post about their lives 6 or 7 times per day, for one thing, they have far too much time on their hands and for another, just how egotistical do you have to be to actually think people are interested in that?
We have also answered your question of AIBU to be absolutely sick of bragging on social media? Yes, YABVVU because you can hide/unfollow/block and unfriend so you don't see any bragging. Simple.
Obviously you are meant to have a sixth sense so that you can tell the boring boasters in advance, so that even those who are "simple" can take action.
So will you answer us as to why you don't if 'bragging' is so troubling to you?
It takes time and a build up of trivial self centred posts to make you realise just how awful a person is. Also, politeness generally means its easier to ignore than take action, unless it reaches extreme values. Of course, the more egotistical will mistake politeness for interest...