All over we say that there are only 2 mitzvos asei that if you don't do them, one is Chayiv Kares- Korban Pesach and Bris Milah. Why isn't eating on Yom Kippur also because there's no lav not to eat but rather a bitul of the Mitzvas Asei of V'inisem?
Sholom Twerski, Phoenix, AZ
Great question. The Rambam demonstrates  that really there is a negative commandment associated with fasting on Yom Kippur, and a portion of his argument is based on your very point.
I hope this helps.
I would add that the main source to help address issue would be in Yoma 81a-b , which the Rambam codifies .