Desktop computers and laptops are designed to be workstations. The iPad was designed to be a toy, and that's how most people use it. That's how Apple markets it, and that's why people buy it.
What Apple and Steve Jobs realised very early in the game is that Americans have a lot of money to spend on toys that look good. Even though most Americans spend their day using computers for work or entertainment, that doesn't make them geeks. They don't need significant computing power, create very little content and only use a very small set of hardware and software resources that are available to them.
The remarkable thing is that most Americans are wealthy enough to spend $500 to buy an iPad. And even though most people could save that money and use it to buy something more useful later, they will spend it on discretionary purchases if the product is considered fashionable enough.