Why are ionization energies always reported with respect to the GASEOUS atom?
Because we wish to interrogate a fundamental process, and be able to compare such ionization energies.
We wish to measure the attraction of the nuclear core to the valence electrons. If we start with a solid or a liquid, then likely we have to heat the stuff up to get it gaseous BEFORE ionization can occur. If we specify the state of the reactants (and products) then we have levelled the playing field, and we can properly compare electronic structure between atoms.