They don’t.
Another post that I wrote just before this one (“Why Your 21MP File “Looks Softer” Than Your 12MP File at 100%“) explains why pixel-peeping* photographers might imagine that cameras with greater photosite density (e.g. – “more megapixels”) might produce “softer” photographs, based on what they observe when they compare 100% magnification images on the screen. These cameras do not produce softer images – the results will either be equal to or better than those from lower photosite density cameras in this regard. You can follow the link to read the original post.
A reader wrote and suggested that perhaps the images from the camera with greater photosite density really are softer, but the cause is a greater susceptibility to diffraction blur or motion blur.
No. Neither is the case.
These are two additional misconceptions that can be fed by (yet again!) pondering 100% magnification crops on the screen without thinking through the actual (non-) effect of what you see there when it comes to actual photographs. Continue reading Myth: Diffraction and Motion Blur Worsen With More Megapixels