re: Why don't NASA's $800 million Mars Rovers have solar panel cleaners?
We don't yet understand what would work on Mars.
For instance a brush or wiper might generate static electricity and cause MORE dust to adhere. Gas blowers might work, or might not.
We need to test the technology, first on Earth in a simulated Martian environment, then on Mars itself, since we still don't REALLY know what Mars dust is like, and how it behaves.
This costs a lot of time and money and it's cheaper in the short term to just oversize the solar panel a bit, and aim for 3-12 months lifetime. After that, NASA runs out of money to man the control room, anyway.
A number of perfectly servicable space missions have been turned off over the years to save money on the ground!