Apart from the vast resources and expensive software and the need for highly trained personnel there is another reason for avoiding too much animation in storybooks, regardless of ethical considerations regarding whether books should even be trying to emulate animated film anyhow, and it is demonstrated in the picture below:
As you can see, it’s a purely aesthetic one. The character in the top picture is beautifully rendered, depicting the light coming from above. The character below is what we generally see in even the high budget animated features, such as those from Studio Ghibli, because the time and resources required to have shadows fall naturally and beautifully upon a moving figure is vast, and impossible to achieve unless moving into the super-duper expensive realm of 3D — which has a distinct look all its own.
Here is a screenshot from Spirited Away:
Notice the backgrounds of Spirited Away are of a different, detailed style than the characters. This is a legitimate and successful style, with the simplified characters looking distinct against the backgrounds — overly detailed characters may well make the screen too cluttered. Characters are instead given form with solid fills of darker colour, with a maximum of two tones per object — Sen’s hair, for example, is one tone for the part of the hair which catches light, and a darker tone to depict the shadow.
A more photorealistic style would depict many shades of brown. When characters in apps are animated, a more ‘painterly’ or more photorealistic style becomes unattainable. This is something to consider whether developing a storybook app or judging a storybook app for its animation, or absence thereof.
But in many works, of course, these two different styles is not an unfortunate consequence of animation but a stylistic decision — as Perry Nodelman writes in Words About Pictures:
Illustrations do have a narrative purpose. They must show us not just beautiful patterns and evocative atmospheres but what people look like as stories happen to them; that is, as they move and talk and think and feel. So their faces and bodies usually have the simplicity, and consequently the expressiveness, of cartooning, a simplicity at variance from the frequent richness and detailed accuracy of their backgrounds, which give us a different sort of narrative information. When faces and bodies do have the same solidity and detail of shading and lines as their backgrounds, they may come to seem static and inexpressive…The extraordinary expressiveness of cartooning seems to make it a particularly appropriate means of communicating narrative information. To suggest that all picture-bo0k art is a sort of cartoon or caricature is no insult; it merely stresses the extent to which the purposes and pleasures of this art differ from those we assume of other kinds of visual art.