One of the problems with using code (besides license and copyright) is the complexity of adapting it into another environment. Sometimes, it fits nicely. Often, it does not. Also, we programmers often have specific optimizations and ways of doing things and 3rd party code may not adhere to these.
Often, reinventing the wheel is easier than not. It gives the programmer a way to add their own custom bits, too (sometimes, improvements).
Lastly, Neolithic combat is probably not like larger, organized battle from subsequent periods. Lots more disorder, ceremony, intimidation. Also, plenty of one on one action/challenges, vs. unit tactics.
