We Doesn't Speak So Good

Last Thursday in Spanish, I had a rude awakening due to many of the issues our class had with grammar not only in Spanish, but in English as well.  While we studied indirect and direct object pronouns, I realized that only about half the class knew what an object of a sentence actually was.  It was very frustrating for me, as I am one of those people who tell you to say "whom" instead of "who," which I can understand is frustrating in its own way as well.  Now I know that no one can be flawless, but I have heard even AP English students remark after a test "I think I did really good on that," and I want to blow my brains out.  Sometimes, I'll hear "Where are you going to?" and I want to hurt someone.  I know this is a touchy subject, because people, for some reason, seem to be very defensive of their bad grammar when I correct them.  My instinct was to paradoxically thank and berate my education in English; in AP English I feel like I have the tools (and the brain) to fix my own mistakes, but I wonder why others have felt like they haven't learned basic rules of grammar, such as when to use present or past perfect tense, or the difference between indirect and direct objects.  I vaguely remember learning something about underlining and boxing different types of words in a sentence around fifth grade, but I wonder why so many people, including myself, have lost this memory (I only remember the rules due to my obsessive need to speak correctly).  Why is it so hard for us to speak properly?