It's also because generative AI relies on statistics so an approximation of the number of fingers is used. In that sense 4 and 6 are close enough to 5.
A third reason is that not all fingers are always visible in pictures and drawings, depending on the angle and position. If you try right now to make a 'c' with your hand, you could only see three fingers.
This is relevant as while artists know that hands have five fingers, generative models do not 'know' what a hand or a finger are.