Here's why:
* Abstract nouns refer to ideas, qualities, concepts, or feelings that you can't touch or see.
* Concrete nouns refer to things you can touch, see, hear, smell, or taste.
You can't physically touch or see beauty; it's a concept, a feeling, or a quality that we perceive.
For example, you might say "The painting is beautiful." You're describing a quality of the painting, not something tangible.