True, but if their top level requirement was to only gather data for 4 years, then they designed the solar panels to simply provide enough power to survive that 4 years. If it survives longer great! But due to their requirements more than likely not being beyond that, they don't need the extra weight.

Remember, they had to maximize overy ounce on the craft to meet the mass limitations. Otherwise they weren't gonnaake it to Mars. Any attempt to add parts to clean the solar panels means they'd have to downsize or outright remove some scientific payloads they brought with which would further limit the landers capabilities.

November 26th is their official 4 year mark since landing, so this dust storm may take it out prior to hitting that specific requirement.

It'll add weight and complexity. More complexity equals more failure points. Also it would require more power storage.

So they design the lander to survive a specific amount of time as a top level requirement. If they don't need wipers (which due to the shape of the panels would only move the dust around on the panels) or any other heavy complex part to meet that top level requirement, then they don't bring it.