If we hadn't added those things, HTML&Cia. wouldn't even be a contender to replace native performance now. It would still be a simple technology meant for static information.

If your argument is more in the line of why do games keep getting bigger and meaner, needing more resources, when we could have stopped advancing 3D a decade ago and every computer would be able to play any game today; then that's a whole other discussion. But I like the path towards true virtual reality with photorealistic graphics.