The answer is a simple one. The US keyboard has fewer keys on the last row so the space bar is longer. All required keys for JIS layout are placed on the bottom row so the space bar had to be shortened to accommodate the additional keys.
Yes, that makes sense, but it would be better if US keyboards had a shorter space bar plus some extra or blank keys outside ALT keys so you can hit the ALT keys more easily from the home position. Maybe I should build some custom keyboards to make it happen!
I use one keyboard with my Windows 10 desktop PC to type both in English and Japanese. Using Microsoft IME, I can switch between English and Japanese with the keys WIN + Spacebar.
If you had a programmable keyboard it would be possible to program this 2-key function into a single keypress. Then you can assign the function to any key you choose. You can also use a macro keyboard to assign any additional functions you wish.