Right, because in the past we were building first stages which had to be optimised for operations in the lower atmosphere, and upper stages which had to be optimised for vacuum. Hence the Saturn V had an RP-1 powered first stage and Hydrolox second stage.

But the raptors on the second stage have to operate efficiently in a vacuuum, but also work well and sea level for the landing. So even taking Mars out of the equation (which we really shouldn't, but just for sake of argument), it's requirements are quite different to anything we've built before.