Here's why:

* Shell Structure: A shell structure is a type of structure that relies on thin, curved surfaces to distribute loads. These surfaces are typically made of strong, lightweight materials like concrete or steel. Examples include domes, arches, and some types of bridges.

* Tree Structure: Trees are biological structures. They use a system of interconnected cells and tissues (wood, bark, etc.) to provide support and transport nutrients. This is very different from the way a shell structure works.

While trees may have a visually similar shape to some shell structures, their internal structure and the way they support themselves are fundamentally different.