Why wouldn't they is the real question.
A lot
Undertale was created by Toby Fox, a game developer for the Mother series. You can actually find him in Earthbound as a dog in the shop in Onett. After Undertale, he later went on to create Deltarune Chapter One. Toby Fox's true identity is yet to be fully revealed.
200+ million
Yes but no at the same time
yes
[object Object]
Undertale
yes yes he did
TORIEL!!!!!!
Sun Flower
Toby Fox. He also I believe he "developed" Pokemon sun and moon, or sword and shield.