Although ceramic can technically be sharper, and technically is harder, it’s both very difficult to get a sharp edge and that edge is extremely brittle and chips easily.

Steel can be made much more precisely easily, and the sharpness has toughness that won’t degrade as quickly. This lets you make an edge that cuts much cleaner, which leads to better shaped particles. Once you start “mashing” the particles instead of cutting them, grind size distribution gets a lot worse and you start getting more “fines” which leads to muddy cup profiles.

Also, because ceramic burrs are used on lower end grinders, the tolerance on the burrs and the other stuff around the burrs (alignment, shaft, etc) is much worse overall which impacts repeatability, and because you are continuously grinding, repeatability (play or shimmy) translates to worse grind size distribution.

Thing is, no grinder can produce perfectly-sized grounds 100% of the time. Due to randomness in how the beans break, there will always be pieces smaller than the desired size. (Probably larger pieces sneak through, too.)

Grinders all produce a distribution, it's just that burr grinders tend to have a tighter distribution than cheap spice grinders. I'd guess that ceramic burr grinders have ever-so-slightly more give than steel, and will let through slightly more undesired grounds as a result.

But it's probably a pretty minor effect.