I don't know why anyone wants to shove big heavy applications into browsers. Are they imagining you'd use your phone for this?

Are we not teaching kids how to publish desktop applications these days or what?

It's just not that we are imagining that people will be using phone to build PCB's, we also have a cli which perform better than the browser playground!

https://docs.tscircuit.com/intro/quickstart-cli

My guess is the cross platform story.

For cross platform development we barely have any decent, free development tools. It's a lot easier to find JavaScript developers in most places than c++/c# developers.