Answer is simple “quantity of steel is different in different structural elements because load (tensile force) taken by different element are different”.
Eg:- Load (tensile force) on beam is slightly greater than load on slab so, steel quantity in beam is slightly more than slab. Likewise in every structural elements.
Therefore,
Slab /lintel——0.7 to 1.0% (55 to 78.5kg)/m^3
Columns_______0.8 to 6.0% (62.80 to 471kg)/m^3
Beam __________1.0 to 2.0% (78.5 to 157kg)/m^3
Foundation____ 0.5 to 0.8% (39.25 to 62.80)/m^3
Note:- In most cases we do not go beyond 4%(314kg/m^3) reinforcement in Column because”it will become difficult to pour concrete and compact it”.