It's mostly due to spell- and grammar-checking programs on computers. Most people assume if they run their writing through the program, everything will be corrected, but that isn't so. Words that sound the same but are spelled differently won't be caught, and grammar mistakes can be inserted. I know this because of my own computer programs.
Also, as everything becomes computerized, more publishers have down-sized their staffs. Fewer people actually read each story or book, and those who do are often over-worked--or never learned to write correctly in the first place, because they've always depended on a computer to catch their mistakes.
