Oilcloth table cloths are the answer. Never leave home without one. Or two.
And yes, it would be great if campgrounds all hit the tables with pressure washers frequently. Almost as great as it would be if campers quit leaving those same tables covered in garbage. Some campgrounds do that. Some campers don't do that.
I've been in campgrounds where it looks like a garbage truck exploded in one or another site. I've always seen staff cleaning up after their guests. And I've always done my part to leave my site cleaner than I found it. Sometimes I feel alone in that mission.