Open access isn't the same as easy access
A blockchain node stores every block, transaction, and resulting state change it has ever processed, and in principle anyone can run a node and see all of it. That's often summarized as blockchains being 'transparent' or 'public,' and it's true in the strict sense that nothing is hidden behind a permission wall. But transparency in that sense says nothing about usability. A node's storage format is optimized for one job: letting the node quickly validate new blocks and agree with other nodes on the current state. It is not organized around the kinds of questions a human analyst, developer, or business actually wants answered.
Consider a simple question like 'how many unique addresses interacted with this contract in the last month.' A node has all the data needed to answer that, but answering it means walking through every block in that window, opening every transaction, checking whether it touched the contract in question, and keeping a running set of addresses seen so far. Depending on the chain and time window, that could mean scanning millions of transactions sequentially, using compute and time wildly out of proportion to how simple the question sounds. The data was never 'hidden,' it was just stored in a shape built for consensus, not for querying.