As a child did you have access to magenta and cyan crayons and paints?
When you are very young and are using paint or crayons, if you need to create a colour that you do not have, then of course you should mix the right proportions of the subtractive primary colours - cyan, magenta and yellow. However, it is very rare that you will find cyan or magenta paint or crayons amongst the selection given to a young child, so your teacher helpfully advises you to use red as an approximation to magenta, and blue as an approximation to cyan. And that's it, from that moment on a young child believes that they can mix red, yellow, and blue to make any colour, and that they therefore must be primary colours. As this concept is instilled in them from such an early age, it is often very difficult to convince them otherwise.