For what it's worth, I have no maths (well, at best I had Maths A level and now I have simple calculus), but my experience from university was that it seemed that most of those selected to study maths (at a maths-oriented college in Cambridge, so this is highly selective) were Maths Olympiads. So it's obviously not just a Singularity Institute thing. In the case of unis, I suspect it's used largely because the general academic qualifications don't differentiate enough for the best people. I suppose the question is whether it remains a useful indicator later on in life.