Back in the days when all brands were made on TV and the biggest ones would be seen almost daily, it was possible to invest in some well-drawn characters and give them a bit of a story. A pretty simple story admittedly, in which their only activity was demonstrating the play value of the product to kids, and its nutritional value to mums.
Decades later they may have been redrawn several times (becoming ever more human in the process), but they stick steadfastly to their task of being the face of the brand. Along with the simple colour schemes, it’s quicker that way.