Ballpoint ink is a lot thicker than fountain pen ink, and is presumably more heavily pigmented. Therefore, a ball point uses a smaller amount of ink to deliver its pigment, compared to a fountain pen. Which means that it uses significantly less ink.

Ballpoints and fountain pen deliver ink to the paper very differently! You can't really fill a fountain pen with thick ballpoint ink because it would jam the feed very quickly. Likewise, if you were to fill a ballpoint with fountain pen ink it would leak as soon as you touch its tip onto the paper.

However, thanks to fountain pen ink being thinner, you don't have to apply nearly as much pressure when writing. Like many things in engineering, it's all about tradeoffs!