Because physiological readiness is based on time, not size. (It takes several months for baby's gut to develop to where it can digest anything but breastmilk.)
Actually, Gerber labels are designed to encourage EARLY solids -- they want parents to buy their products as soon as possible. The AAP specifically recommends waiting until 6 months to begin, even if baby can sit up with support a bit sooner than that.
Consider too that if baby is a big boy, then he's obviously THRIVING on milk alone. So why feel the need to add anything else to his diet? At best you'd replace a complete food with an incomplete one. At worst you'd add more food and more calories, and make him gain weight even faster. (A few bites of cereal aren't going to make him take less formula.)