In humans small differences in weight are usually not that big of a deal. Other factors like liver function or wether you take an oral drug with or without food introduce much more variability. Even if weight varies greatly between subjects, it’s often because of a higher fat mass. Most drugs don’t distribute into fatty tissue that well and so in those cases a weight based dose might over dose someone. Fat free mass is sometimes used but often not really needed. In children we do use a weight based dose though.