Most of the answers above are nonsense.

One reason you must get your butt into the seat is so that folks don't simply scan fares and buy the cheapest from anywhere to anywhere to collect the miles. Do you think any airline wants to sell me every $149 transcon seat they have thus allowing me to buy miles and status for next to nothing?

Here's an example, in the above scenario I buy 100K RDM and 1K status on UA for $2500. But after reaching 2P I got a 25% RDM bonus, and after reaching 1P I got a 100% RDM bonus. So I really have more like 175K RDM.

So, now for $2500 I have enough RDM for 2 C class tickets to Europe plus change and 6 SWUs (not to mention CR1s, 500 milers, etc.). Net value maybe $10,000 without leaving my computer.

Now do you understand why you need to fly to earn miles?