To simulate the fly's neural input, you need to simulate the fly's entire environment, including fluid dynamics for the air around the fly, the physics of every object the fly interacts with, hormonal responses to changes in the flies blood concentration of various substances (O2, glucose, ...).

This already strains our technical capabilities (at least for the amount of money we are willing to spend on it).

For any animal whose behavior is the product of a lot of learning, and who deals with other such animals in daily life, you have to solve all the problems you had to solve for the fly and also deal with a much larger connectome and also deal with the fact that, until you can passably simulate mouse, you will never be able to simulate the way a mouse learns to behave in the presence of another mouse.