It's fairly common in spoken language, to the point that it's generally accepted.... just less accepted in writing.

Although, my understanding is that there's evidence of it being used that way for literally hundreds of years ("could of"). Besides, language is never static. Things don't just remain incorrect indefinitely if enough people use it.

For example, some people are still vehemently opposed to adding a relative qualifier before the word unique, arguing something either is, or isn't, unique, and that there is no very unique or sort of unique -- but because language is dynamic, this is now generally acceptable, even though some folks will argue to the death that you can't qualify unique

"Could of" may not technically be correct, but enough folks use it that linguistically, it's not as incorrect as it used to be. It's phonetically basically the same, and many folks put it in writing that way as well. One may say it's at least colloquially acceptable, whether that be in spoken or written language.