This is the case with anything you didn't discover it yourself but rather 'learned it', that's actually just a replaceable term for 'memorized it'.

People who discovered/invented these algorithms traversed several years of curiosity, blind alleys, dead ends, failures and successes. You can't possibly expect to get it all and make it up on memorization alone.

Algorithm learning is hard for the same reasons memorizing Math tables are hard.

As of today there is nothing novel about merely learning how a algorithm works and spitting it out in an interview, if anything it achieves the exact opposite goal of what it is supposed to.