I think most research code bases are used by a significantly smaller number of people. Hence it's easier for the researcher to know what's going on in their cod

I think most research code bases are used by a significantly smaller number of people. Hence it's easier for the researcher to know what's going on in their code because they probably wrote a significant portion of the code they use and hence understand the assumptions, and limitations that are baked into the code. Whereas if you try to use a library normally you have to kind of figure those things out on your own. That said, having used a lot of research oriented software packages over the years, researchers typically do not produce high quality software. Their objective is to produce papers, not software that other people can use. If someone else can use it too, then great, but that's not the primary motivation.