Buying two identical drives has high chances of them being from a single batch, which makes them physically almost identical. It’s a pretty well-known raid-related fact, but some people aren’t aware of it or don’t take it seriously.
Normally, failures come from some amount of non-repeatability or randomness that the systems weren't robust to.
The drive industry is special (in a bad way) in that they can exactly reproduce their flaws, and most people's intuition isn't prepared for that.