Why are keyboard numpads on the right-hand side?

All standard computer keyboards have numpad (numeric keypad) on the right-hand side. Why is it so?

I have recently switched to a laptop without a numpad. This lets me have the mouse in a much more ergonomic place on my desk. Mouse is an essential piece of equipment and it's used much more commonly than numpads (which are nowadays often missing on keyboards and most people don't miss them). So why not place numpad on the left-hand side, so that the main part of keyboard is centered with mouse on the right and numpad on the left?

What's more, numpad is useful mostly for entering large amounts of numbers, for example in a spreadsheet. With numpad on the left, I could simultaneously enter numbers with my left hand and navigate in the spreadsheet with mouse/cursor keys.

I've googled for keyboards with numpad on the left. They are uncommon and advertised as dedicated for left-handed people or "mouse-friendly keyboards". I'm not sure if left-handedness really matters here. For example right hand is my dominant one but I actually prefer using left hand while typing, so I could happily use the "left-handed keyboard".

That being said and assuming that most people would be as happy with left-hand side numpad as I would (bold assumption, maybe?), what are actual disadvantages of having numpad on the left? It's more ergonomic and healthier, potentially more functional.

Why are numpads on the right-hand side?