Why do raindrops fall with constant speed during the later stages of their descent?

The gravitational force is the same for all drops

All the drops fall from the same height

Air resistance just balances the force of gravity

The force of gravity is negligible for objects as small as raindrops

As a raindrop falls:

Initially, it accelerates due to gravity.
As its speed increases, air resistance (a drag force) also increases.
Eventually, a point is reached where the upward air resistance equals the downward gravitational force.
At this point, net force = 0, so acceleration stops, and the raindrop continues falling at a constant speed.

This constant speed is called terminal velocity.