Apple also use AWS, apparently two years ago
spending $30 million per month. They used to leverage Azure as well but
picked up Google back in 2016.
They invest but there is a level of pragmatism of outsourcing bulk storage to third party vendors. It makes more sense to pay AWS and Google to store those chunks of data and have them be responsible for additional replication. Depending on how Apple has structured the pieces, having copies on two independent clouds allows for handling outages in a single vendor (including Apple themselves if they handled storage) and for the other clouds possibly locating data in regions closer to the user (e.g. Australian user data for iCloud stored primarily in Australia to provide a better experience for those users rather than round tripping to somewhere in the US). Setting up global data centre presences is complicated and expensive, why not outsource the easy block storage?