Why do capillaries need to be thin walled?

That's because one of the functions of capillaries is to transfer things such as oxygen, carbon dioxide, waste, or similar things, between cells. A thin wall enables these transfers to go easier because diffusion of them is easier. If the walls were thick then the process would last longer.

We can use solar powered cars or ride bikes instead of cars that burn fossil fuels

The direction a chemical reaction occurs is represented by an arrow