As someone who graduated recently and is going through this process right now - I always thought I would be set because I'm a decent problem solver, aced all my algorithms and data structures classes and generally could solve most interview style problems I came across. I've learned that this is not enough. An organic problem solving process might involve trying several promising approaches, or starting with a suboptimal algorithm and realising improvements to it, and then you might figure out an optimal solution. In many of these interviews the time constraints can be absurd, you basically have to have done the questions or variants of them recently to be able to write down the optimal solution in your first iteration.
Today my friend had a first round online screening from Atlassian - 5 questions in 90 minutes, and none of them were trivial warm up level questions. Compound this with the fact that it's often harder to solve problems and think creatively when you're under time pressure in an interview, and you realise that your only option is to do 200+ Leetcode problems and just hope your interview overlaps with those problems.