Probably part of why (the other obvious reason being die space) Ryzen lacks 256-bit SIMD hardware, and instead runs such instructions with a 128-bit unit over two cycles.
AVX2 might have also been just a bit too late to be integrated outside the microcode.
As usual, actual achieved clock speeds depend on thermal performance and are not fixed. If cooling is completely insufficient, the clock frequency will drop below base clocks, i.e. thermal throttling.
To clarify: This isn't a secret, but part of the specs and even in some Intel slides. (However, platform and processor specs, even at a basic (product D/S) level, tend to be a rather lengthy read, so most do away with reading all that).