Why Feed point impedance is set by the antenna's own geometry and its electromagnetic surroundings. Height above ground is a big factor because ground reflections couple back into the antenna, so a horizontal dipole's feed point impedance swings well above and below the free-space value of roughly 70 ohms as height changes, converging near that value at a wavelength or more up. Nearby objects and the antenna's length and diameter matter for the same reason.
Watch out Transmission line length and tuner settings change the impedance seen at the transmitter end of the system, not the impedance actually present at the antenna's feed point; power level is irrelevant because the antenna is a linear, passive load.
Feed point impedance is set at the antenna, by its surroundings. Anything back in the shack only changes what you see, not what's there.