I imagine the answer to "why are flowers beautiful" is probably linked to a different question: why do humans have superior color vision vs. almost all other mammalian creatures?

A dominant theory is that natural selection favored individuals with extended long wave (red) perception. Better color perception enhanced locating high-quality food resources and avoiding toxic plants. Human vision also resolves finer detail than many mammals.

Flowers are worth noticing, after all, they're precursors to fruit which may be edible when ripe. Appreciating the form and color of flowers isn't a random phenomenon. In its present-day form, like most things human, the "beauty of flowers" is a complex phenomenon arising out of interacting biological, social and environmental factors.