Why is it that Ethereum source code is not found on Github where people can actively contribute to it and also why isn't Vitalik Buterin actively working on the source code on Github or am I missing out on anything? Their competitor "Cardano" is open sourced and anyone can submit a PR to actively work on the project which I feel is very beneficial to the community.
I am not sure why you think Ethereum is closed source? Do you mean certain protocol updates?
As far as I know Ethereum is fully open source. Otherwise it wouldn't hold that much value. Nobody in his right mind would put billions into the network without knowing whether the source code contains some backdoors.
As for updates, I don't know how transparent they are about that during the research and coding phase.