This is a really good question. If benzene can react with the protonated methylcyclohexene, why can't it react with H2SO4? I believe the actual answer is that it does. However, the reverse reaction results in the reformation of benzene, thus it will be a no-reaction reaction.
If you were to treat benzene with D2SO4, the result will be deuteration of benzene. In this case, the protonation-deprotonation steps result in a different compound.