Because you don't hire firefighters. Senior devs should be able to reason about complex problems and come up with good solutions after a few hours or even days,

Because you don't hire firefighters. Senior devs should be able to reason about complex problems and come up with good solutions after a few hours or even days, depending on the problem at hand. You test their skill of solving trivial problems in 10 minutes under pressure. I can accept if people defend that as the best proxy measure they have for the amount of effort they want to invest into each candidate. But do you seriously believe that this proxy filters the good senior devs from the bad ones?