I can think of two reasons for this:
- There is a lot of work that goes into creating actual, renderable animations from the table structures defined in the data phase. This work is done once during load, so you can't make any animations at runtime. I guess the only way around this would be if you could pre-generate a set of animations in data, and then swap references to them later.
- Prototypes can't change at runtime, partly because this would have complex and inefficient implications for every instance.
- Creating multiple copies of a prototype, each with one of the animation variants I want. Then swap entities out for other ones entirely instead of just trying to swap their animations. I could see this being a potentially computationally intensive task (that scales with the number of entities), and there might be some work involved in carrying over each entity's inventory to that of its replacement.

