I don't get it and some people have tried explaining but it seems to ultimately boil down to "I just don't like/want to" which to me is incredibly insufficient. These people would rather throw away perfect food even when they made it themselves. Leaving out certain exceptions that don't keep well, many dishes can even taste better as leftovers while others can be combined to form entirely new dishes.

Edit: to those asking, this is about several people in my life who throw out a lot of food because they don't leftovers and when I've asked them about it I've felt that they don't really give good answers. These are people who afaik don't have eating disorders since they eat normally but will not do takeaway at restaurants or save extra portions of a meal they've cooked.