IMacs, all-in-one PCs and branded computers in general are designed with planned obsolescence in mind. These systems generate constant waste with their upgrade cycles, which only consist of complete replacement of outdated systems.
Custom-Built PCs rely on standard components only, which are generally much higher in quality than those included with branded systems and are usually more cost-effective. The components in this slide show are some examples of the premium quality found in a Custom-Built System.
You will also be happy to know that when these components become outdated they can be sold to regain some of the money you spent on them, or they can also be recycled into other computer builds.