Where the arm enters the gearbox casing (bell housing), the steel shaft rotates in plain aluminum.
This is the part that sticks, as the arm is angled downwards slightly into the bell housing, allowing water and dirt to run down it, eventually corroding the aluminum (aluminum oxide)
Only really a problem if car is used infrequently, though regular lubing with thin oil, or spray grease every service helps prevent occurrence.
Also there is a seal on top of the bell housing, to stop this happening. How many people actually change this when the clutch arm is removed?
If it is getting as bad as you say, then be prepared for the clutch release bearing to fail!
peter