They've been told a blogger should blog often--every day if possible. So they fall into the trap of throwing something out there.
They tell themselves a post doesn't have to be very good--it just needs to fill the space so they can say they blog every day. Consequently, their posts are unremarkable.
Casual Communication? Yes, But Good Too
Then there are the bloggers who think a post is a casual means of communication--kind of like jotting something down on a sticky note.
In their opinion, a blog post is just a top of the head way of communicating with the audience so it doesn't matter if it is full of grammar errors, typos, incorrect punctuation, wrong words, and misspellings.
Readers Avoid Those Blogs
Reading a blog like those described above is arduous at best, if someone makes the effort. Most visitors won't make the effort. I don't.
Readers just move on to another blog. Why waste time reading something incompetently written when there are blogs written by articulate authors with a mastery of language and grammar?
Bloggers, respect your readers. The blog is a tool that can work for you or against you.
Vow to publish content that educates and/or entertains, and that you'll make that content easy to read with good grammar, punctuation, etc.