I have had the same experience. With coding I can be in that coveted state of flow for 10 hours straight. With creative writing, I feel exhausted after 3 or 4 hours. I didn’t expect this and don’t fully understand why.
There are a couple of reasons as far as I can tell. Programming can be adequately gamified or fulfill basic dopaminergic feedback loops in a way that “true” creative expression (outside of an effort-action-reward feedback loop) cannot. With fiction, the value is strictly non-instrumental or context-specific. While the parsimony of the (formal) language you deploy can be (reflexively) satisfying to look back on, the real joy of art for its own sake is the response from the reader, which is necessarily slower than that of a computer, as it requires a more complicated system of evaluation than pure utility.
