Because if it is a simple harmonic vibration, there should be no external force except the restoring force, but the resistance of the pendulum (friction and air resistance) is inevitable, so in fact, the pendulum always has external force to compensate for energy consumption.

But if it is forced vibration, strictly speaking, there should be periodic driving force, but this is not the case for a simple pendulum.

If you have dismantled the pendulum, you will find that the top of the pendulum is actually an interesting gear structure. Every time the pendulum moves backward and left and right, the gear moves forward one grid, and then it will knock on the top of the pendulum to give it a thrust to compensate for the energy loss. This is why the pendulum ticks.

As for how to define this swing, it's up to you. Anyway, it's definitely not what you said. But if you do a rough calculation, or explain it to others, the pendulum is actually quite ok. After all, the damping of a pendulum is very small.