Cheaper than building your own datacenter, or buying your own server space and paying a dedicated engineering team to monitor a cluster for you. I can do things on my own or with a small team that would be well out of my reach without AWS or GCE.
It depends on the point of view, for startups looking for a tech solution at a small scale to test their products, these solutions are usually much cheaper than doing everything by yourself. It's only when you start to scale that you realise the financial impact and the lock-in that the platform have upon you, and at this stage it's too late for the paradigm shift to operate.