Blockchains aren't about currencies. They're about providing a single immutable scroll for a society to write entries on. You can use a scroll to record transactions for a currency, but you could also use it for any interaction between people.
Associative arrays aren't about phone books. They're about providing key value storage. You can use an associative array to record phone numbers, but you could also use it for any key value pairing.
Why the same hash table for two different things? Why the same block chain for two different things?
Blockchains aren't plain old data structures. Their value comes from consensus, and it's way easier to use an existing consensus than to establish a new one.
It is. Establishing a new blockchain is a hard thing to do. Ethereum also benefits from network effects between contracts: any contract can call another one to use its functionality.
