Why? Aside from the excellent reasons given by others below, remember that even conceptually simple systems can and often have surprisingly complex behaviours.
The emergent complexity of interacting simple systems is... often breathtaking in the scope of how whacked out the unexpected can be.
tl;dr - "simple" programs do not necessarily have "simple" behaviour or "simple" interactions with other "simple" systems.
Make sure the damn thing works before simplifying it.
Why? Aside from the excellent reasons given by others below, remember that even conceptually simple systems can and often have surprisingly complex behaviours.
The emergent complexity of interacting simple systems is... often breathtaking in the scope of how whacked out the unexpected can be.
tl;dr - "simple" programs do not necessarily have "simple" behaviour or "simple" interactions with other "simple" systems.
Make sure the damn thing works before simplifying it.