The reason is quite simple and obvious
Decentralized storage use less money to operate their servers compared to centralized storage the examples you have mentioned are all big names large corporations are expensive because they are operating on a large scale hence why their services are much more expensive than the centralized ones
I believe the primary reason is not the cost of the service, but rather the marketing. Google, Amazon, Dropbox, and Apple invest billions of dollars to lure users into subscribing. Almost all of them end up utilizing less than what they pay for, such as 30% of a 1TB subscription.