Photoshop allows you to build interface designs in a layered workflow. This allows you to build realistic designs, that look and feel like they do in real life. Whilst a button may seem like a simple thing to build, it takes time and energy to do it well, along with some attention to detail to make things look realistic.
Planning is the key to any success, just like in the real world you wouldn't build a house without a set of plans. The same set of rules apply to building interface designs. It ain't no five minute fix...
