There's no calculation a GPU does that a CPU cannot, and in the very old days the CPU just wrote to a particular location in memory when it wanted something to show up on screen. The reason you need a GPU is that displays have millions of pixels which need to get updated tens to hundreds of times per second, and GPUs are optimized to do a whole lot of the operations needed to render images all at the same time.

It's sort of like asking why we need container ships when aircraft exist that can carry cargo, and the answer is that the container ship can move a whole lot more cargo at once, even if it has some restrictions on where it can take that cargo.