Why does a sample of cobalt-60 not visibly lose all of its mass as it decays?

A sample of cobalt-60 does not visibly lose all of its mass as it decays because the unstable cobalt-60 nuclei transform into stable nickel-60 nuclei, which remain within the sample alongside the still-undecayed cobalt.1

While radioactive decay involves a fundamental change in the nucleus—often converting one chemical element into another—the process does not cause the original material to simply vanish. Instead, the daughter atoms (in this case, nickel-60) persist in the sample. Although there is a minute loss of mass associated with the energy released during the decay process, as described by the mass-energy equivalence principle ($E=m{c}^2$), this change is not perceptible to the human eye in the volume or total mass of the sample.1234