Countless reasons.

Invent a gui client environment that is universal, works the same way everywhere, over any kind of channel, and is already implimented and supported everywhere, and utterly weightless in all dimensions (ram/cpu/network), and then you might be able to ask that question without it being incredibly ignorant.

Today the closest you might be able to say is web/electron, which is gross on all counts. If anything the closest is X11, which is not remotely close enough.

That's why not just...