They created the "math" behind closed-doors. The methodology behind these technologies is not fully disclosed. Sure, the technology is open but not the process it took to create it which can make all the difference.
What exactly are you implying about the process and why is that relevant? Whether Newton derived calculus on the toilet or while sitting under an apple tree or had a 'process' of drinking a cup of water every 30 minutes is irrelevant to the mathematical proof.
First I will note DSA relies on hash functions such as SHA-256. The mechanics of Calculus are not equivalent in complexity to the workings of a hash function. It would be arrogant to say that the complexities of the SHA-2 algorithms are fully quantified and that they are uncrackable. Do you understand what it entails to analyze and understand cryptography? Do you understand the resources it involves? It's not just auditing regular source code.
You can't just put it front of one expert and be assured confidence. Holes in cryptography can take many years to find. One can deduce such holes can be well hidden.