Sorry, but I can't give a better answer than that.
Typically the number keypad does not have additional characters, (the ones you access from the number keys on the keyboard proper by pressing Shift), so may be that has some bearing on it.
Or, possibly there is another factor. Some users will need these extended characters often, and using a number keypad is like using the old calculator or till machines that used to be used in accounting offices or in shops. It is more convenient for fast access under just one hand with the other simply pressing the Alt key. No good for me though because, although I am right handed, I always used my left hand for these calculator machines and my right hand doesn't 'remember' as fast as my left does!
That's the best I can do!
Here's a technical question for you. I just discovered that to type a symbol using the alt key (like alt+0189 for 1/2) you have to use the number keypad and can't use the top row of numbers. I was helping my husband type in a degree sign on his laptop and he couldn't do it unless he holds down the blue fn key & the alt key and types the number using the keypad keys. Tried it on my desktop, same same. Never noticed that before.
Why can't you use the number keys on the top row above the letters?