Mathematicians attempt to express ideas in the most readable and clear way possible.
It's actually code that must be obfuscated by the constraints of the language and computer. Mathematicians have no constraints preventing them from presenting something in the way that makes the most sense.
The part that can be called "Obscurantism" is when they use a high-level abstraction you are unfamiliar with. This is mostly driven by the audience.
> Indeed, 99% of pure math is non-constructive
Citation needed?
> the algorithm converges in O(N) steps
That doesn't sound non-constructive. Even the group example is usually done by constructing such a group.
Also the best part about math is you can use it to approach the problems you want with constraints you want. Knuth uses math to solve real CS problems.