You need to keep in mind that the rovers also needed to survive the six months in transit in once piece, and the airbag-assisted landing in one piece, both of which add necessary reliability and durability factors to make sure that the thing lasts even the 90 days. Also, you need to keep in mind that those cleaning events came out of left field. Yeah, they were expecting to maybe get a few extra weeks out of the rovers, but because the wind behaved in a manner that was completely unexpected, the panels weren't as dust-caked as was expected, which had always been the real limiting factor.
Such as what, exactly? I'm sure it was considered, but it's not that simple. Would it involve air blasts? Liquid? A squeegee? The sand/dust content wasn't exactly known so a scraping motion would potentially scratch the glass surface and make things worse. Compressed air would take a tremendous amount of energy to compress and a liquid is heavy.
