That's like asking why you'd rent an existing house instead of purchasing a pre-made one, or building one from scratch.
What are your skills? If you can build a house, can you build a better one than the ones available for rent? If not, why would you build your own?
What are your requirements? Do you have 0 children? 3 children? 7 children? Does the amount of people staying in your house vary a lot? If you rent, you can just move to a bigger or smaller house so that you're never too low on rooms and also never paying for an empty room.
Some folks *could* do a better job at the infrastructure level than Amazon. By some, I mean a list of companies that probably could be counted on one hand. Google, Microsoft, IBM, Apple, maybe some others, maybe some of those don't make the list. If you work for somebody not on the list, Amazon is probably going to be better at this than you. Amazon won't be perfect, but they'll be better than you.
Many folks have no idea what their usage is going to be. Is the next game going to be a hit? Is the social site for the TV show going to have heavy traffic during airing hours and none otherwise? Is the business seasonal? For many use cases, we're talking about a house where you mostly have a couple with their child, a few hours a day you have 10 children, and then every so often you'll have a few thousand over. If you're building your own, you'd just have to WAY over purchase capacity in order to avoid the occasional slashdotting. If you're renting, you just launch a bunch more app servers, then release them after the wave. You pay by the hour, or tenth of an hour
What folks are saying is that this isn't a silver bullet. Just because you're in "the cloud" doesn't mean you don't have to think about service levels, redundancy and all that. You still have to do your job - figure out your in-house skills, your budget, your requirements, and solve the problems. It's just that, if you know what you're doing, you can use the cloud to save yourself a fuck-ton of money.