the zero is large and right next to the thumb, the numpad is rectangular and better fits the hand, and the arithmetic operators are far more accessible and don't need the shift modifier to actuate.
What this guy said. The number row is just too far above home row to be comfortable to use, unless you shift your entire hand up, but then the problem is things like +, - which need the shift button which is super uncomfortable to use if you're hands are up on the number keys as home row.
Also you'd be relying on you're weakest finger (the pinkys) to actuate the important keys you'd need in this set up like shift, plus, minus, equals sign. And then there's the issue of multiplication and division. Multiplication would either be x button all the way down there on the bottom row or the * button which is another shift modified button. Division probably would be the / button, another out of the way button especially if you're typing on the number row. It's just a bad, bad, bad, set up.
If you ever took a business class in high school where they taught you tenkey, you'd see just how much faster it is, espeically when dealing with mathematical/computational situations.
Lastly, tenkey is the set up for a lot of counter machines like cash registers. It's a standard already, part of it being that it saves space (can you imagine a long row of numbers at a cash register?). This is also why tenkey was standardized, as another poster said, so the other hand can do something which someone at a cash register needs to do often.
EDIT: Note this is coming from a guy who has a tenkeyless board lol. For home use I don't need it but what I pointing out here is the reasons for why it exists and as a purely numerical input device, it's better in a lot of situations if ALL you're gonna be putting in is numbers.
