Because after a successful, productive, 5-10 years at a company you tend to forget the algorithms you don't use. I can remember enough about the knapsack problem to talk intelligently about it. I can't remember how to do the dynamic programming. In 90% of cases given infinite time I can eventually arrive at the solution to most med-hard questions. However, you don't have infinite time. I find problems like optimizing how much water goes into buckets to the point I can solve it in 4 minutes with variations so far below me it doesn't even register on my radar. For the record, I have over a decade in this industry. Having to do these problems makes me feel like that hard earned decade may as well had not happened.

For those of us with lives, hobbies, etc grinding leetcode is exhausting. Moreover, you can't just grind it. You have to grind it to the point you literally can brain dump answers in an interview.

Interviewers rarely care about your accomplishments. No one cares you did staff eng. at XYZ, managed engineers, have familiarity with all sorts of frameworks, etc. This is the only industry where every time you look for a job you're treated like don't have a resume. I think many (if not most) people are insulted by this because the people in charge are belittling them. I would be even Knuth would struggle to tackle a modern programming interview. That's a problem.