AKshuaLly, the biggest reason for not using a full sized keyboard is that it's just cheaper.
On average those extra numpad keys that you dont usually use, cost $15~20 extra for a keyboard enthusiast. (and it can go more expensive)
The average keyboard enthusiasts also often build around 6 keyboards before they get bored/moved on to different hobbies/run out of money, so that's on average $90-120 saved
Reminder that the most common variation, the TKL, is literally just a normal keyboard without the numpads.
There are other variations yes but only psychos use them.