Indeed, as anyone who can Google would discover. But when did WebGL become readily available for Scratch users to use in browsers ? That's the mode of use which Scratch 2 is specifically designed and intended for.
In my book there's little good which comes from designing something which its target audience cannot readily use.
But what they did, and whether they could have done it differently seems rather immaterial to where we are now. I know very little about graphics frameworks or hardware acceleration, and much of the rest is 'greek to me', but, from what I can ascertain -
WebGL is based on OpenGLES 2. WebGL 2 is based on OpenGLES 3. The Pi's VideoCore IV only has support for OpenGLES 2. So, for WebGL 2 - which is what it appears Scratch 3 is using - that cannot be accelerated and has to be done in software.
That could correlate with jamesh's comment that future Pi's may better support Scratch 3 more fully than current generation Pi's do.
Maybe someone who does understand more about this field than me can clarify and educate ?
And, just because there may be no current support for accelerated WebGL 2, perhaps no chance of that with the Pi's we currently have, it does seem that things could be improved if Scratch 3 were doing things somewhat differently.
It seems the main problem is that Scratch 3 is continually rendering its Scratch 3 'stage' area, so, if it did less of that, the CPU usage for rendering should be less.