C and C++ libraries with light language-specific wrappers largely serve this purpose, in practice. It's plausible that, say, a PHP postgres client lib and a Node postgres client lib will share nearly all their code—probably as C or C++.
This doesn't get leveraged much aside from interfacing with daemons and sometimes for extremely complex e.g. media or crypto libraries, though. No-one's doing this for high-level software workflow building-blocks, like a password reset flow.
[EDIT] as for the "why", I suspect it's because the things it's used for are far simpler than the things it isn't. A password reset flow can potentially need to interface with lots of different things, some of which may be custom to the project, some of which may vary with run-time input, et c. If you cut it down to only the parts that could truly be re-used anywhere, with all kinds of points where you can hook in as needed... you've only made about 5% of the work re-usable, so it's just about pointless.