Worth noting that while AlphaGo and AlphaZero are incredible achievements, the amount of actual code to implement them isn't very much.
If you have the research paper, someone in the field could reimplement them in a few days.
Then there is the large compute cost for training them to produce the trained weights.
So, opensourcing these bits of work without the weights isn't as major a thing as you might imagine.
There is Leela Zero (https://github.com/leela-zero/leela-zero) for Go and lc0/Leela Chess (https://github.com/orgs/LeelaChessZero/repositories) for Chess, where both provide trained weights. The Leela Chess project specifically have been working for a long time on training and refining the weights for Chess, as well as providing the code -- they allow you to see the history and performance over time for the various trained models.