Why does natural selection act on phenotype rather than genotype of an organism?

By definition, the phenotype is the actual physical manifestation of an organism's genotype, so the way an organism looks or behaves is what is being selected on. For example, if ladybugs have red (R) shells or yellow (r) shells and ladybug-eating birds only eat the ones with red shells, any red-shelled ladybugs will be selected against based only on their phenotype. The birds do not know whether each bug is RR, Rr, of rr, so phenotype, not genotype, is what is affected by natural selection.