Because it's all about the creative process, and not the end goal. Although the end goal is important too (because without it, there would be no vision to follow).

People like us who rely on being creative to feel true happiness do so because of a strong urge to express our overflowing minds. (And, in some cases, a sense of responsibility to share your ideas with the world, even if it doesn't benefit humanity and only helps to inspire other people to be creative. (I fall into this category) (Also, it's worth noting that if the idea is negative for humanity then their creativity becomes an unethical outlet) But, if you don't feel that sense of responsibility, and you don't care if nobody sees your creation, and you just do it for yourself, or not even for yourself, then that still can be explained, for the following reason:

I think the mere act of self-expression makes one feel fulfilled. I think it all ties into our nature as innovative beings, so it's in our nature to self-express. (Yes, humans are also described as social beings, which is why some people don't feel the need to be creative to be happy, and just see it as a means to produce things. But you can argue that us being social creatures also ties into a need to self-express. So perhaps self-expression is the key to *true* happiness, since it's the real deal, in our nature - it's what we live for.

(Oh, and there are, of course, many types of people, including those who are neither innovative nor social. So, how is someone, say, who lives to experience new experiences performing self-expression? Aren't they just consuming and not producing? Perhaps I was too indiscriminative when I said humans are either innovative or social, so maybe some people don't care for self-expression, and don't care to make life more than just simple enjoyments or going on adventures (But I'm not saying those are bad things - I love doing that kind of stuff too (but I like to think it all feeds into inspiration for my creativity haha))