Per (Genuine) curiosity why ? I know (good) database systems are hard to make but do sqlite have particulary 'hard' part which are near impossible to replicate ? Especially since we now have a (I assume) well documented base implementation and an extensive test suite/history of error to avoid ?
Is making à file based database that hard,compared to creating new programming language for exemple ?
Yes. I can hack together a trivial compiler in a weekend. A serious (non-optimising) compiler for a complex language is a task you could do in a few months.
If you gave me two year I'm not sure I could implement a filesystem API that correctly did something as simple as "atomically append to a file". See also Dan Luu's writing on the topic of filesystems: https://danluu.com/deconstruct-files/
This reliability comes from a very deep knowledge of OS API and all their corner cases. Creating a programming language typically involves less corner cases and the bugs in the compiler/runtime are much simple to reproduce (again, typically).
Sqlite is one of the most battle-tested pieces of software in the world. It's used "everywhere" and it's performant.
It is not the case, as some seem to believe, that rewriting software in Rust will magically eliminate all bugs and give a free speed boost compared to well-tested and optimised C.
So, yes.