The radiation resistance of a half-wave dipole is \$\small 73\Omega\$. This relates directly to the feedpoint impedance, i.e. this is the impedance presented to the transmission line by the antenna at the design frequency.

The impedance of free space is the impedance presented to the antenna by its environment, which is a different kettle of fish. There is no reason that this impedance should be directly transferred to the feedpoint.

The radiation resistance, \$\small R_r\$, of a half-wave dipole is \$\small 73\Omega\$. This relates directly to the feedpoint impedance, i.e. this is the impedance presented to the transmission line by the antenna at the design frequency.

\$\small R_r\$ is related to the impedance of free space (i.e. the impedance seen by an E-M wave travelling in free-space), but is not equal to it.