A door is the hardest-working opening in a house. It gets opened thousands of times a year, it carries its own weight on hinges, and its threshold sits at the one place where water collects.
Humidity swells wood doors and warps them out of shape, which is why the door that closed cleanly in February drags in August. Fiberglass and steel do not move the same way.
Threshold rot is the most common failure we see, and it is almost always an installation problem rather than a product problem: no sill pan, or flashing lapped the wrong way.