They're not trying to build a sustainable business. They're trying to get as much market share and lock-in as possible before the bubble bursts. This makes a ton of sense from that perspective. It probably would be cheaper for them in the long run to own their own hardware, but they are paying AWS for their expertise so they can focus on what they do. If it doesn't work out, it also sets them up for a merger with Amazon.

I do think a ton of businesses would benefit from running their own hardware, but they're not getting five billion dollars to stay on the cloud.