I've sat at the gate many times and watched my plane come in, passengers deplane, crew deplanes, the plane sits for awhile, the catering truck pulls up, baggage gets loaded, crew boards then my flight is boarded. I'm sure someone empties the trash and does a cursory check of the plane, but there's definitely no time for a thorough cleaning. I've found sticky rings and crumbs on my tray table before. Sometimes there's something tucked in the seat pocket. I never see visible trash or messes when I board. I've learned to take a pack of wet wipes and do my own cleaning. I wipe the tray table, armrests, air control, etc. It only takes a few seconds and at least it will be cleaner than if I didn't do it at all.