Multiple reasons:

1) you can't get new hardware delivered and get it up and running, all in under 10 minutes.

2) also, assuming the previously gathered hardware is not needed anymore, you can't just return it and say "i used it only for two days because i had a traffic spike, take this 200$ and we're okay.

3) you can't programmatically install, configure, reinstall and reconfigure hardware configurations, networking and services on phisycal services. At least, not as easily.

Many others, but these are very valid points.

Of course, Amazon is not the solution to all of the problems you could ever have, but still it solves a great deal of problems.