Why are the $d$-block elements known as "Transition" elements? Why are zinc,cadmium,and mercury not considered transition elements?

(N/A) Originally,the name transition metals was derived from the fact that their chemical properties were transitional between those of $s$- and $p$-block elements.
According to $IUPAC$,transition elements are defined as elements that have an incompletely filled $d$-subshell either in their neutral atom or in their ionic (oxidized) state.
The presence of incompletely filled $d$-orbitals makes transition elements distinct from non-transition elements.
Zinc $(Zn)$,cadmium $(Cd)$,and mercury $(Hg)$ have fully filled $d$-orbitals in both their neutral and oxidized states ($d^{10}$ configuration).
For example:
${ }_{30} Zn: [Ar] 3d^{10} 4s^{2} \implies Zn^{2+}: [Ar] 3d^{10}$
${ }_{48} Cd: [Kr] 4d^{10} 5s^{2} \implies Cd^{2+}: [Kr] 4d^{10}$
${ }_{80} Hg: [Xe] 4f^{14} 5d^{10} 6s^{2} \implies Hg^{2+}: [Xe] 4f^{14} 5d^{10}$
Since they do not have an incompletely filled $d$-subshell in any common oxidation state,they are not considered transition elements.