And it's not only the gravity but density advantage. delta vee = v_ex times ln (mass ratio). It's the mass ratio part. The problem for first stages is that som

And it's not only the gravity but density advantage. delta vee = v_ex times ln (mass ratio). It's the mass ratio part. The problem for first stages is that some high ISP propellants are very non-dense. Liquid hydrogen is 70 kg per cubic meter. Kerosene is about ten times as dense. So the tanks need to be a lot bigger for the same payload. The same holds for pumps and turbines too. So the engines need to be bigger for the same thrust. Hydrogen is also annoying to deal with as it's much colder than liquid oxygen, the molecule is also much smaller and leaks much more easily. Technically the fueled all-up hydrogen rocket might be lighter, so the engines should be smaller, but the density disadvantage eats back all that advantage. The dry mass, which is the actual cost driver, is about the same. For upper stages, it makes sense because the cost driver is not the dry mass but the wet mass. Also they need less thrust so engine mass is a smaller proportion of stage mass.