Very few people on the planet face a threat model where memorizing a seed phrase is worthwhile. Leaving a seed phrase accessible long enough to memorize it considerably increases the likelihood of it being compromised. Locking it up and hiding it immediately greatly reduces this possibility.
You are right about it not being difficult to memorize a seed phrase. It is not hard to do many things that are unnecessary and unlikely to be useful. Only an idiot would fail to realize that such an exercise is essentially pointless, and then proceed to brag about his ability to memorize a few words, while simultaneously demonstrating his inability to reason about basic things.