How come it seems that so many people do not know how to use "THEN" and "THAN" properly?

Update: English is not my first language, but I was taught those two words and how to use them properly, since I was in the 2nd or 3rd Grade.
Update 2: @RedFox: Yes, their, there, & they're as well as to, too, & two. Are schools in the West really that bad? I understand that people may misspell words, especially, the uncommon words that are not used frequently; but it seems that so many people do not even know how to use those words properly in a... show more @RedFox: Yes, their, there, & they're as well as to, too, & two. Are schools in the West really that bad? I understand that people may misspell words, especially, the uncommon words that are not used frequently; but it seems that so many people do not even know how to use those words properly in a sentence at all.
Update 3: @roy: I am referring to grown adults, especially, in the U.S. and Britain, who were born & raised in those countries (I travel to those two countries quite often for work).
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