Re: "create function... depends on extension..." not supported. Why?

Note: I’m asking because the answer to the question “Why isn’t X supported?” is always useful. For example, if supporting it would imply a logical impossibility that I’m too slow to spot, it helps me when someone explains what I failed to realize. Equally, it helps me to know when the answer is “It’s just a historical accident. It could have been supported. But, now that it isn’t, it doesn’t seem worth the effort to bridge that gap” because this shows me that my existing mental model is sound.