Building a superhuman AI focused on a specific task is more economically valuable than building a much more expensive AI that is bad at a large number of things.
It also comes with ~0 risk of paperclipping the world — Alphazero is godlike at chess without needing to hijack all resources for its purposes
I’d recommend reading Stephen Wolfram on this question. For instance: https://www.wolframscience.com/nks/p315--the-intrinsic-generation-of-randomness/