Nobody who cares about their tools leaves them outside when they are not in use. Damage from moisture and sunlight can quickly degrade the condition of a tool, and what could have been a lifetime purchase turns out to be something that needs replacing over and over if it is not cared for properly.
Extension cords deserve the same consideration as your most prized tools, and if they are thought of that way, you can get a lifetime of use out of them.
Exposure to moisture can lead to corrosion, which can limit current carrying capacity. Long term exposure to sunlight can cause cable jackets to become brittle and more prone to create shock hazards.