At school I was taught multiplication ("times") tables up to 12, and there it ended. Maybe because it is/was a commonly occurring number?

It occurs commonly in the Imperial system, for sure. But I was brought up in the age of metric, and we were only taught the times tables up to 10 in school (but I learned the next few anyway, because I just love the times tables). Coincidence? I think not!

So why not stick with decimal systems?

Ah, the age-old question we ask ourselves, every time we read a USn first-year physics textbook, which inevitably has a few problems written in Imperial units! Like, who wants to convert miles per hour to metres per second? Honestly! Or square feet to square metres? Yeeeech!