Mathematica has an army of mathematicians working on edge cases that only other mathematicians would care about. It's essentially impossible for any open source software to match what mathematica does.
(https://reference.wolfram.com/language/tutorial/NIntegrateIn...)
That's why nothing can compete. Not only does Mathematica have an extremely complete implementation of common functionality, it even has an extremely complete implementation of very niche functionality. I don't think that the people who propose OSS solutions really appreciate this, probably because they aren't professional mathematicians/physicists