It comes down to, "What percentage of energy output from the panels needs to be devoted to keeping them clean?" If it's 100% there's no point in going to Mars, if it's 0% there's no point in using solar panels.

At least back with Opportunity the solar panels did have wipers. They ended up lasting much longer than expected. I'm not sure whether Insight has wipers or not, but it's possible they shifted too far in the other direction.