If I would write my own data storage I would re-implement SQLite. Why would I want to do that?
Not sure if this is quite what you are getting at, but the SQLite folks even mention this as a great use-case: https://www.sqlite.org/appfileformat.html
SQLite is kind-of the middle ground between a full fat database, and 'writing your own object storage'. To put it another way, it provides 'regularised' object access API, rather than, say, a variant of types in a vector that you use filter or map over.