[Rusty]

Most English words are a single syllable, which works very well in songs - one note for each word of the song. Sometimes, the meter of the song just won't abide a two-syllable word so, for the song's sake, we abbreviate and blend words into a single syllable. That's why "ain't" is so popular in songs; it's a single-syllable word that means a number of different phrases that would normally take three notes to sing!

For example:
I ain't got no ... (sung in 4 notes, but it is non-standard English)
I don't have any ... (proper English requires one more note to sing the same thing)
I do not have any ... (singing it this way requires yet another note)
I haven't got any ... (ditto)

So, songwriters choose the non-standard English phrase in order to fit the meter of the song. This is known as 'poetic license'. UNFORTUNATELY, the non-standard phrase is quickly embraced and before we know it, it's accepted as everyday English. Grammarians have died many times over in the last 4 decades alone!

We Americans love to shorten our words, and we blend them together, if possible, to make even less work for our tongues. Colloquial speech is MUCH different than what is taught in the textbooks. I've noticed this happens in all languages, though, so we've no need to feel like we're alone.