Just need to input fewer calories than you use. A body at rest still uses calories to maintain basic life. So just keep them hydrated and add certain nutrients that aren’t in stored fat. Unfortunately your body can also use muscle as stored energy. If you are inactive, it will burn muscle more than you’d like. It’s expensive to keep around. So if you’re on starvation diet and not using them. No reason to keep them. The body burns them up. So in the end you’ll be lighter, but also extremely weak. The effort of rehabilitation may be as bad as trying to exercise and diet not in a coma.

The biggest issue would be muscle atrophy. Even if you're pretty sedentary, your muscles are doing a lot of work each day just to move you around. Being bedridden for even a few days can cause muscle loss. You also increase the risk of blood clots and bed sores.

Medically induced comas also come with risk of permanent injury or death, so they really only do them in absolute emergencies.

And lastly, there are already surgeries and medications for weight loss that work much faster. The issue is, without matching lifestyle changes, all that weight will just come back.