Ledgers are quite easy to build. Git is a ledger. It doesn't use a block chain but it does link commits via their hashes; which makes it an audit proof ledger. Unless you can break the hashes, you can't modify its history.

And git is used between many different developers and companies working on the same projects. It was actually designed for this use case. The reason git doesn't need blockchains for this is because it funnels all the commits and pull requests through trusted code owners: people. The consensus model is very simple: a mutually respected developer has looked at the thing and gave their thumbs up.

Banks, insurers, etc. use ledgers. But mostly they lack cryptographic safety. And they use a similar reputation based consensus model. Auditors, regulators, accountants, etc. It's good enough but the failure modes can be somewhat expensive. And of course the flaky nature of the whole system actually represents a business opportunity in it self. Banking is a very lucrative business.

Blockchains are potentially a nicer/cheaper technical alternative for all this. But so far not an essential or very practical one. And of course most block chains have scaling challenges. Ethereum is a non starter for running anything at scale. Transactions are too slow, too costly and the throughput sucks. Even with the new version it's still nowhere good enough to run even a small bank. Highly unsuitable for running millions of accounts on where the cost of each transaction would far exceed the value of that transaction.