Because joy is different from happiness.
Happiness is that wonderful, fizzy feeling you have when things are pretty much going your way, you’re doing what you like, and you’re smart enough to be grateful for it.
Joy is a sense that the grounding of things, the long-term outcome, the big picture, are whole and positive.
Happiness is great, but it tends to fade in and out, like an iffy signal on A.M. radio. Joy, on the other hand, hangs around.
When the prose stinks, the characters fall flat, the plot meanders into ever deeper and more stupid places, writing brings me joy, even though I may not be so happy right then.
So, I persist, and when happiness shows up to accompany joy, well, that’s a great day.
I wish you many days of both.