That's because it is memorizing chess openings, or basically the same thing.

Most Leetcode Mediums fall into the category where the type of solution is obvious (sliding window/graph search/etc), and the code isn't hard to write, but there's a "trick" embedded in the problem and if you don't already know the trick, you're never going to figure it out in 30 minutes.

So the best strategy is to find a "Top N Leetcodes" list like Blind 75 and just memorize them. Of course you can't expect those exact questions to come up in every interview, but having a few dozen solutions already memorized should let you do some pattern recognition and lower your cognitive load while you're trying to figure out what that problem's trick is.