Why there are no sharp corners in nature?
The premise of your question is entirely wrong. There are many examples of sharp corners in nature, the most obvious being well formed crystals.

Pyrite is a classic example that was mentioned by @wienein in the comments. In a ideal situation it forms cubic crystals with sharp edges.



A classic non-crystalline example would be volcanic glasses such as obsidian, which fracture to produce extremely sharp edges and were historically used by some cultures for making blades and other sharp tools.