Why quantity surveyor don't deduct volume of steel from volume of concrete?

Due it's minor percentage which is ranging between(1-2.5%) of total volume of any constructional  member, beside that there is an extra quantity of concrete should be added in calculation, inclusive the wasting in quantity  during casting process.  

Because steel volume is comparatively to concrete element that coneys is minor and can be neglected. around 2%

Since the diameter of the steel rod compared to the area of concrete in a section is negligible i.e a beam may have size of 200mmX300mm but the diameter of the steel rod is 12mm(Assume) which is negligible when compared with 200mm or 300 mm.         

Consider a 200x200 column with 4 nos of 20 mm bars.Total volume of steel: 3.14x0.02x0.02=0.01256Total volume of concrete= 0.04-0.01256=0.038744Almost same as 0.04

We do not deduct volume of steel from volume of concrete, Because the volume of steel in concrete is very minor from1-2%

steel is the part of concert, in reinforcement of  concrete can not deduct the steel RCC

in plan concrete can passably to deduct the concrete   in volume 

As compared to the volume of concrete, the volume of steel is very less to neglect. In standard practice volume of steel will not deduct from volume of concrete.

Deduction of steel from volume on concrete does not permit in the civil engineering method of measurements rules. Concept behind is the difficulties to make form work and pure concrete cannot established easily due to steel thus it will compensates this efforts. Also volume of steel is normally ranging 5-10% which is minor.

Moreover, in KSA this deduction is being exercised.

Standard practice is that no deduction shall be made for the volume of any reinforcement or structural metal enclosed in the concrete. However, kindly refer to the preamble stipulated in your contract BOQ.