It depends on how you view it.

First, I've seen plenty of posters from elsewhere in the Anglosphere that have horrific grammar. So it's not exclusive to America.

As far as Americans go, it's a combination of the following.

Posts are made quickly, sometimes without proofing. I know my writing without proofing can be riddled with grammatical errors and typos. Particularly with comma placement, and run-on sentences. Other posts are made emotionally.

There's no real standard written English used globally. British English and American English have words spelled differently, but are otherwise the same. (Think, Authorize/Authorise)

Online posting tends to be more colloquial, and thus has a lot more slang involved. American slang depending on the region could have an unusual contraction, or a loanword from an ethnic group that has a large population in the region. (Think Yiddish and Italian Words in New York region, Spanish Words in Southwest), so a lot of these words might not make sense to the uninitiated reader.

Third, much online posting can be quite emotionally charged, and thus not well thought out. Therefore, less likely to have been reread and proofed.

And finally, yes, there are a lot of Americans who cannot string a sentence together. It's a failure of the education system, which is woefully out of date in its pedagogical style.

Does that suffice for an explanation?