The cpu is a handful of college math majors, they are skilled in handling a wide variety of problems and in general are much faster than most at doing those calculations. The gpu is a gymnasium full of 5th gradrs, don't ask them to handle advanced calculus but give them a few thousand basic algebra questions and that mob of students is going to be done way faster than those couple of grad students.

Less eli5 : In general the cpu is deciding what happens on the screen and the gpu is in charge of saying that that looks like. As the one takes a lot of varied calculations and the other is more specialized at just drawing shaped and applying textures to them, but doing it with a ton of cores at once.

When it comes to games the cpu is running the game itself, saying what entity is where and where things are headed. The gpu is constantly trying to draw what the cpu says is there, it loads all the physical assets into its own memory, does all the calculations for how the scene is lit and dumps its result onto the screen. Once it's done with all of that it asks the cpu where everything is again and starts all over.

The cpu contains only a handful or so very powerfull general purpose core to do the work, a modern gpu on the other hand has thousands of less flexible dumber cores that can brute force their way through all the work it takes to generate frame in a modern game. Plus having much faster memory on board the card itself helps when gpu is constantly referencing large texture files and storing information dealing with the current frame its working on.