You're assuming the launch vehicle can carry the weight of both the tank and the payload. Really if you want to maximize the tank size & weight you put into sp

You're assuming the launch vehicle can carry the weight of both the tank and the payload. Really if you want to maximize the tank size & weight you put into space, separating the two is the only way. To steel man your point, Starship could be looked at as one giant ass tank with a small payload?