Concentrated $H_{2}S{O}_4$ cannot be used to prepare HBr or HI from KBr or KI because it

$H_{2}S{O}_4$​ when concentrated act as oxidizing agent and oxidizes KBr and KI to Br₂​ and I₂​ respectively.

$$\begin{gathered} \mathrm{KI}+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{KHSO}}_4+\mathrm{HI} \\ \mathrm{HI}+{\mathrm{H}}_2{\mathrm{SO}}_4\to 4{\mathrm{I}}_2+{\mathrm{SO}}_2+2{\mathrm{H}}_2\mathrm{O} \end{gathered}$$