Because thats a totally arbitrary statement and it's also not really true. It's pretty trivial to swamp the GPU, even if you know what you're doing. If you want to show something big/complex or very pretty, or even just novel rendering that will stress the GPU (raymarching, scattering), the CPU will be waaaaay less burdened.

CPU bottlenecking is an issue in videogames where the main loop is usually handling a massive amount of computation, doing its own intersection checks, etc. Etc. Plus any other processes queuing sounds, running physics etc. There are just a few cores doing a huge amount of work.

Its still relatively easy to swamp the CPU in javascript (webworkers and async obviously help), but if youre just piping orders to the GPU then any CPU can easily max out the abilities even on high end cards. In most cases that's basically what webgl is used for, AFAIK. How many full on ai and physics heavy games are there? Webgl games tend to be lighter, and are very often accessible ways to play with interesting shaders. The use case tends towards GPU bound.