Selective Grammar Fail

I was listening to the song If She Would Have Been Faithful by Chicago today.  It got me wondering why no one says "she would have was" or "he would have was" - At least I've never heard it. Have you?

• She is
• She was
• She would have been

Somehow they get that right. 

Why is that? People are always saying "I could have went" or "she had went" or "I seen" - all of which are wrong and sound just as wrong as saying "she would have was."

• I go
• I went
• I could have gone

• I see
• I saw
• I should have seen

Did no one pay attention in school?  Or did the teachers just not teach?  Maybe everyone was absent on the days they taught could have gone and I saw.  There must be some explanation.

Have you ever heard anyone say "I must have did" either?  Except Luke Bryan.  I mean, sure, did rhymes with kid, so maybe it was necessary for the song -

I was a rock star, party hard,
Getting over you comeback kid
Yay I must have did
What all my friends say 

but should there be some rule that songwriters should at least try to use proper verb tenses?  Or maybe it's just artistic license, and everyone else is just living life as one big artistic experiment.

I like language better than art.  It makes sense, and it's obvious when something is out of place.