When we calculate the quantity of RCC.why steel quantity not add Concrete

You can do that if you desire. For example, in a 3 foot cube ( one cubic yard of concrete), if you had 3 rows of one inch rebars both ways, the volume of the steel would be about 0.67 cubic feet, or about 2.5% of the total volume of the 3 foot cube. For small jobs, it is insignificant and some waste will be experienced because the producers work with minimum amount orders etc.
For large quantities and/or very heavily reinforced sections (say 100 cy), one can save several hundred dollars by considering the volume of the resteel but the savings have to be weighed against the risk of running short at the end of the pour, which would negate (probably by far) any savings.