Writers are only human and human beings make mistakes. Here some common reasons why writers might publish errors in their content.
Keep in mind that there are many reasons why writers can make grammatical or spelling errors and leave them in.
1) The function of the human brain
I wrote an article on that in Bukisa. You can read it here. So that is the main reason we read over our typos: our own brains can fool us and that is why you should not feel guilty for using a spellchecker.
2) Eyes in the back of you head
Yes, writers mostly write series of articles and after a while staying focused is also fatiguing.
Editing is also a matter of copy/cut and pasting for many modern writers who use keyboards. A wrong mouse click in the end might also do some harm.
3) Non native writers
Many writers here use English. For many this is not their native language, so they might make translation errors while thinking the content in their own language.
4) On line translation
Some non native writers might use on line translation programs. These do not really produce the most readable text, since they do translate word by word and do not mind the English grammar.
Also does a dictionary not always provide the right word when you look something up.
For example: a windowsill in Dutch is a “vensterbank” or literally window bank. Well, some programs will give you “window bank” as result. Well, bank is actually more correctly couch, but that will take it too far.
5) Common used errors
Some expression are so commonly used wrong, also by native speakers, that we all might adapt them.
For example: something costs much or is expensive, but when in a certain region people keep saying something costs expensive, which is wrong, but almost everybody says it that way, you might make that mistake as well.
6) Pronounced the same, but written different = different meaning
Words like whether, weather or wether can be mistaken, especially for vocalizing writers, or writers that read their texts aloud to listen how they sound. That is why creative writers might suffer more from this and a spell checker might not always get it.
Did you mean loose ends or did you lose ends. And you can easily read over it.
Are or Our? Sounds the same.
If you see a mistake in an article, think logic and you might also read over it. Keep in mind that the writer might have read over it several times. Do not bash, but send a nice comment with the corrections, if you see them and like them corrected.
Do not question the writer. Keep in mind what you should have learned from this article.