Even free google photo storage is not unlimited. If you exceed 15GB of Google drive space while storing photos in original pixel density and quality, you’ll end up having to pay for more storage. So if you’re shooting pictures at 16MP or more, or a lot of 1080P or higher resolution videos, you’ll end up paying for google storage. And google charges exactly the same monthly rate for 200GB storage as Apple does - $2.99/month.
While Amazon Prime does offer unlimited storage, you are of course paying for your Amazon Prime membership. And AWS benefits from being the single largest provider of cloud computing services in the world. Everybody uses them to some level for their own services, even Google, Apple, IBM and Microsoft.
Cloud storage and cloud computing costs the companies offering it billions of dollars to set up, and hundreds of millions annually to operate (Apple has invested more than $10 Billion in the last decade or so to build out icloud). So of course there is no free lunch with cloud based services. Beyond some basic introductory storage and services, every company charges for cloud storage and cloud computing - Apple, Google, Amazon, DropBox, Box, Microsoft, IBM - everybody charges for it since it costs them huge amounts to continue to provide it.