Why the primary colours of paints are different from the primary colours of light?

Because color in terms of wave-length is an additive phenomenon, and tint, in terms of pigment is a subtractive phenomenon.

can someone explain that a little simpler, please… sorry, i’m not that smart :]

When you mix paint, blue and yellow don’t really make green. The absorb (subtract) everything that isn’t green. When you project a blue light and a green light together, the length of the blue wave is added to the length of the green light.

Primary Colors in terms of light. Can be tweaked together to make any color.

Pigment color needs separation or the end result will be black.

Totally different type of color v light models—transmitted light vs relected light.
Sum of all pigment colors is black absence of all is white.
Some of all light-based colors is white. Absence is black.
Although pigments in actuality fall far below the concept—all colors mixed is usually some muddy dark browny color.

BTW primary colors used in the printing process CMYK [cyan, magenta, yellow (and black or K)] actually are opposites of light-transmitted colors RGB [red, green, blue]. Cyan is opposite of red and contains green and blue. Magenta is opposite of green and contains red and blue, and yellow is opposite of blue and contains red and green—that last may be hard to swallow but try light through red and green gels and see what color it makes.

Also these things are standards in art and graphics but it’s not perfect science, the human visual spectrum covers a wide range and CMYK RGB are just the most efficient way to reproduce most of it with the fewest elements. For example you can’t reproduce neon with pigments or ultraviolet with light, etc. and when painting artists use a lot more base colors than just RGB, which work ok for dot-printing but get muddy when you mix them for paintings.