The marginal utility of the second marshmallow is lower. Then there is added cost in the time spent waiting. Even if you have to wait in the room regardless, I think it’s fair to say there is a cost to delaying the gratification.
Why one extra? What if I just give you 5% of a marshmallow? 1%?
A coherent argument can be made that the extra costs are not worth it. I don’t particularly like marshmallows, but maybe I’ll eat one. Maybe two marshmallows in the future would be higher utility or whatnot than one marshmallow now, but that isn’t always the case.