12 column grids is a convention and it is just that, it's not a standard.
The reason it exists is cause 12 can be divided by 1, 2, 3, 4, 6 and 12. So it is very flexible.
There are 10 and 16 column grid systems as well which have their own advantages. The initial versions of Bootstrap used to have a 16 column grid. You can create your grid logic in any way you want. It all depends on what you're trying to achieve.
But needless to say the 12 column system is easy to understand and works well for most layouts, and thus, is more popular. It only makes sense for a designer to know the quirks about it.