I think they're designed by very traditional engineering managers. The coding test trend gained popularity thanks to Jeff Atwood because he used it as an early screen for people applying for lucrative jobs they didn't actually know how to do (which is useful!). Managers were using it as a higher and higher floor for skills and we got the leetcode style (a fresh bootcamper might pass fizzbuzz but they're unlikely to have months to grind leetcode). We've also seen an explosion in roles who code but are more responsible for the wisdom and value of their creations (the spec and visual design aren't enough or even relevant for a model, a Lagrangian relaxation, a recommendation engine, etc).

Real conversation I had with another executive, "Hey, we've got that coding screener for engineers, can we whip up something similar for [name a role]?" You start combining these different forces - a desire for selectivity, a desire to lower hiring cost, more complex technology roles that have to chart some of their own spec, and just human laziness - and you get what OP described.