Most programmers can't produce large quantities of high-quality pixel art. So for a game that uses a scrolling map, it is wise to enlist artists. Ordinarily, it goes like this
- Develop an engine
- Make a demo
- Attract artists
- Make the game itself, as a total conversion mod of the demo that you produced in step 2
But in order to make the demo, you'll need some art assets to put in your demo.