Comment by jerf

Because when people try it, it doesn't look or work very well.

I think it's important to understand that this doesn't mean that such things will never work, but it is also important to understand that almost every idea that you've ever heard of has actually been tried lots of times (lots of lots of times in many cases), and there are often good reasons that they haven't actually been adopted.

As others are already pointing out, the linked proposal bears a striking resemblance to what Smalltalk does, so it's more helpful to ask "Why haven't the many attempts at this approach been successful?" than to ask "Why hasn't anyone tried this?" The first may lead you to a successful variant, the second will lead you down the same garden paths that everyone else went down.