Because you can only play games with your tama when it weighs over it's base-weight. So if you feed it one snack it will gain weight and you can play with it.
it is when it plays it normally loses 3lbs and if its base weight is 20 and its weight right now is 20, if it plays, it will lose 3lbs and then its weight will be 17lbs which is below its base weight and cannot happen. the same thing with if it is at 21lb,or 22lb