Indeed. It doesn't have to consistently work, be easy to modify, be efficient, be well documented, etc., and in general usually won't be since there is no reward for any of these. It just has to "prove a point" (read: provide sufficient support for the next published paper, with paper reviewers caring far more about the paper's text than any associated code or documentation).

Anyone who spends lots of time trying to make research-relevant code projects with solid architecture / a well designed API / tests / good documentation / etc. is doing it as a labor of love, with the extra work as volunteer effort. Very occasionally a particularly enlightened research group will devote grant money to directly funding this kind of work, but unfortunately academia by and large hasn't found a well organized way to support these (extremely valuable) contributions, and lots of these projects languish, or are never started, due to lack of support.