I think the two subjects are very different. Math has a much longer history. To get to the edge of knowledge you need to know a lot more because people have built proofs upon proofs for much longer. You need an undergrad plus a year or two of grad to get both the depth a breadth of what’s been done in the past.

In CS the edge of the field isn’t as far away. New knowledge invalidates more of the old, and you don’t even need a CS undergrad degree to be near some of the edges.