The symmetric keyboard is a fun concept, but its existence as niche is similar to why QWERTY is still dominant. Inertia. Most keyboards have it, and computer keyboards are much easier to change than pianos. Once you have reached a certain fluency, the jump needed from this local optimum to a new one is prohibitively high/far. For musical instruments, this would mean you would be unable to play anywhere but at home.

That's a huge drawback and it's really underappreciated by everybody advocating for the "better" concept.

Besides, there is the unrelated drawback that especially for a beginner, it's really easy to learn simple tunes with just the white keys on a piano. Throw in a black one now and then and you can get quite far and have fun as a kid. This would be much more intimidating with a symmetrical layout.