I think the problem is being expected to regurgitate* 1-2 hyperoptimal leetcode solutions in 45 minutes while suffering from heavy interview pressure.
*by regurgitate, you can't simply implement the optimal solution either even if you know it. You have to put on a show where it seems like you're arriving at and iterating towards the optimal solution, "explaining your thought processes". But you can't waste time iterating and going down suboptimal paths either because you only have 45 minutes or less. Hence why ideally you know the exact optimal solution beforehand, or at least know the tricks and patterns to quickly get to the optimal solution for the type of problem you are tackling.
Recruiters and official interview guides say that your "thought processes" matter a lot, but reports from in the field tend to imply that the #1 most important factor is that you get the optimal solution. If you can't, your "thought processes" are worth little, barring exceptional circumstances.