Because spelling words correctly lends credence to the writing. In other words, poorly spelled threads, and your thread is a great example, tend to make the readers think that the writer is either illiterate, ignorant, a troll, a kid playing on a computer, etc. Yes, the occasional typo is forgivable, but a thread with every third word misspelled is not only hard to read, it's also hard to view as credible.
It's an indication of the substance of the content and the intelligence of the poster. Your case proves the point.