How do you know? Where did you come up with evidence to confirm that statement? There has never been a platform as large as Twitter that opened up its algorithm.
As an example, the bits of Google's original algorithms that they did talk about (pagerank using reputation of in-links to propagate to a page) publicly began getting gamed into irrelevance fairly early on (link-for-link schemes being the start of the SEO industry). After that it's been a continuing struggle of a whole industry looking for weaknesses and Google adapting.
Another way of thinking about it, is that if someone could see what features were most important for the ranking on some site, then they could start to optimize for those, breaking the usefulness of that feature. One obvious example of this is "Please remember to like, comment, and subscribe" on YouTube.
For a long period of time, Reddit used to be open source. It can be done, plus additional changes to add transparency on top, like giving people the option to switch between different recommendation algorithms, making it clear why content was recommended, and so forth.
