Why tenderfoot second class and first class rank do not require any merit badge?

Scout, Tenderfoot, Second Class and First Class ranks are designed for young men to learn the basics to take them through the program and live up to the goals and ideals of scouting. Much like a sponge, in these early ranks you suck up all the information, so as you go to higher ranks you can turn from taking in, to giving back. This is why the other ranks require service, and leadership. Merit badges are designed to expand that base of knowledge, and while they can be earned at any rank, are then applied when you reach Star and above.