Why In a grounded-grid (cathode-driven) stage the drive signal is applied to the cathode, which is a low-impedance electrode carrying the full cathode current. That gives a typical input impedance on the order of tens of ohms, which is convenient because it roughly matches 50-ohm coax drive from an exciter. The grid acts as a grounded electrostatic shield between input and output, so the stage is stable without neutralization, but the driving power passes through to the output, so power gain is modest.
Watch out High power gain is the tempting opposite: grounded-grid amplifiers actually have less gain than grounded-cathode designs because the drive power adds to the output rather than being isolated from it.
Grounded grid = cathode driven = low Z in, low gain, no neutralizing needed.