property of conc. H2SO4

It's all about equilibrium.
H2SO4(l) + NaX(s) <======> NaHSO4(s) + HX(g)

While sodium halides and sodium hydrogensulfate are solid, sulfuric acid is non-volatile. Hydrogen halides are volatile, and vaporize rapidly in concentrated solutions especially under elevated temperature.

When mixed, HX are continuously removed from the reaction mixture, driving the equilibrium position to the RHS and yields more product.

If HX are not volatile, they would remain dissolved in the mixture and are not coming out and extracted.