Do you have any idea of the chance of rolling 4 ones on 4d6?
It's somewhere around 0%. More accurately, it's close to 0.077%. Odds are, you'll roll that sum once in 200 throws (or there abouts).
Once in 1,296 per ability score.
Once in ~200 per character.
Once in ~36 per party of six characters.
So if 36 groups make characters using 4d6 drop lowest, one of them (on average) is going to have a character with a 3. I think there are more than 36 groups who make PCs using this method.