Ive usually wondered if they were typing so fast that they just DIDNT HAVE TIME to focus on stupid picky things like punctuation and sentence structure and spelling and life is just too short and u have no WRITE to tear me down for it and
It makes my head hurt, too.
I do try to be charitable about it. The poor poster may have sick or crying children, and/or may have just finished talking with that uncaring spouse or that ubiquitous nasty MIL. She's upset, she's probably exhausted, and she's possibly pouring her heart out in the ninety seconds she has - on a very small cellphone keyboard.
I'm pretty good at writing, but I always, always have to go back and double- or triple-check to see if anybody will be able to understand what I write. My brain knows what I want to say, how to say it, how to spell it, and even how to punctuate it, but my fingers sure do mess up. It takes time to go back and do that sort of checking.