Getting rid of third party's code often isn't as easy as removing a folder and be done. Sometimes third party code is deeply integrated and was patched and can't be isolated or it might be the core component/core engine where the missing make aThe reminder almost useless. Also some vendors put their APIs behind an NDA, thus you not only have to remove the API, but also rework the calling code. (Just for open sourcing fighting whether the NDA is legally clair is quite some effort ...)

This also becomes fun if you worked with external contractors, who committed all over the code and the agreement doesn't cover releasing the code.

And then there are still all the other things in open sourcing (making sure comments are appropriate, naming is appropriate, ...)

And then you have IP besides the source, in artwork where designers often have some rights, copyrights, trademarks ...

And after all that the "fear" on your reputation, if one fears the code might seem of low quality, which might reflect bad. (Whether it's truly bad doesn't really matter, a few blog posts or youtube videos of somebody taking worst parts can harm reputation)

It's all simpler, if you plan on doing that eventually from the start, but notable effort for any serious project after the fact.