It has become so commonplace for people to spell check, but they do not check grammar. Therefore, the system the person is using may not catch the wrong word, because it is the "wright" word (spelled correctly, wrong usage) and since the system didn't flag it, the person's brain (if they are proofreading it) accepts it as the right word.
We have gotten lazy in teaching proper grammar and word usage. Sometimes, the wrong word will slip through, when texting a message or writing anything on a computer, because of spell check apps that are not grammar apps and a person hitting send without really reading what they typed.