Because of browser support honestly.

Outside of grid-layout, there is nothing salvageable from the CSS layout engine that cannot be done better inside a programmable view such a canvas.

Give it 5 years or so for WebGL/WebGPU to standardize, and new UI libraries with a better layout engine and UI constructs far better than what can be built on top of HTML/CSS will show up.

Nobody thought we would take HTML/CSS as far as we have, but it has already served its purpose.

Your comment reminds me of this article back then: https://engineering.flipboard.com/2015/02/mobile-web