A large enterprise project will need security, testing, auth, (AI now too). I'd hate to implement SAML without a library, that would be torture, and likely incompatible with most systems.
While I've often written small self projects from scratch, I wouldn't dream of building a large one that way unless you are sure to have an army of engineers and QA.
As an aside, this is where AI code fails as well. Speed of dev is easy, stability over time and compatibility is hard.