Don’t get me wrong, I appreciate art more than most. But there’s an exclusionary aspect that exists with art, wherein only some people can truly appreciate various aspects.
In contrast, nature is more universal and primal. Everyone, regardless of language or culture or education, can appreciate natural phenomena. The beauty of nature speaks to us on a fundamental level, whereas the beauty of art requires a certain degree of acculturation and intellectual effort to grasp.
Furthermore, human art is a reflection of nature and indeed a part of the beauty of nature, as you say. However, that inevitably positions it as a subset of the all encompassing beauty of existence as a whole. Artistic works are small mirrors reflecting back aspects of reality in interesting ways. But because they can only ever represent fragments of the greater whole, they are somewhat less awe inspiring.
Often, works of art can prompt us to engage with the beauty of reality, so I’m not condemning them in any way. I’m just saying that the representation can’t be better than the real thing, even if humans wish that it were.