The in the very definition of 'economics' explains why. Since, economics is a social science that studies human behaviour, is a relationship *BETWEEN* ends and scarce means..
'ends' that is goals, and "scarce means" referred to the resources. So that is the economic problem, that there are *unlimited* wants but scarce resources. So the answer to this question is just: All goals cannot be reached because there are unlimited "goals", and few/limited resources.
