And weight budget. At the end of the day they can only send what they can launch and land in one go currently. So if you spend 1 kilogram on a system to clean panels that is 1 kilogram where you don't have a science instrument. It is also like 100 kilos in propellant to get that 1 kg to the surface.

So yeah it is usually better to just try other means of keeping them clear that may extend the life but may not.