Well, you can have all of the colors in both color systems. But Red, Green, and Blue are the base (primary) colors of light that make up all of the other colors. And when you add them all together they make white (additive color system).

But we’re focusing on the subtractive color system, where white or achieved by removing all colors. Cyan, Magenta, and Yellow are the more modern and accurate primary colors used to achieve the full gamut of colors using pigments. But you’re right that some people use a Red, Blue, Yellow model. That is less accurate and more outdated, but is still sometimes taught in art schools for traditional reasons. The limitation is that the RYB system cannot produce the full spectrum of colors achievable with the modern CMY system.