Offering unlimited storage does tend to require some sort of investment. Sure, you can throw some large HDDs into a server and offer 'unlimited' storage to users but there is a finite amount of space in that machine and when it runs out what will the provider do? Beyond that - do you really want to be on a small array of large spinning disks? Probably not.
Most quality providers offering unlimited storage have separate storage platforms such as a SAN or a distributed cluster storage platform. This allows them to add storage to the platform and to assign it to any server without worrying about how many drive bays a particular server has.
Another benefit in most cases to providers that do this right is that they don't so much have to worry about storage available to individual servers and can focus on the big picture - how much space is available on the cluster as a whole. Additionally you generally gain more redundancy against failure as most of these platforms keep multiple copies of the data on multiple machines/drives.
That being said - a provider that is doing this right is not going to be able to offer unlimited at anywhere near $10/year.
Storage costs money. Quality storage costs more money.