There are a few main reasons. For one, your baby’s muscular coordination is still in the works. While the hands and feet are being progressively more refined in their controlled, the muscles required to control the vocal cords require even more refinement before they produce intelligible language. The fact is that it requires a lot of coordination to say words such as the movement made by the tongue, lips, mouth, and lungs. However, to make signs, a baby can do much simpler movements. This isn’t to say that a baby will make perfect sign or one even a complicated one, it just means that they can get away with sign approximations much sooner than they can get away with speaking.
At first your baby will use grunts, and squeals, and might point to help show others what they want. However, this grunting and pointing can be refined using more abstract associated signs like bringing the fingertips together for MORE, or twisting the closed fist against the cheek for APPLE. In fact, baby’s as young as 4 months old have been known to understand signs and babies as young as 6 months have been shown to be able to use signs to request specific things.
So while non-signing babies are left pointing and grunting, signing babies can make specific requests. So while non-signing babies might be poorly understood, signing babies can make their wants and needs made perfectly clear. This produces more confident children and parents!