Other answers above have told you just how insecure brain wallets are and how humans are a terrible source of entropy. Now, even if you were to generate a properly secure BIP39 seed phrase, here is why you should write it down and not try to remember it.

Each year:

69 million traumatic brain injuries: https://pubmed.ncbi.nlm.nih.gov/29701556/
12 million strokes: https://www.world-stroke.org/assets/downloads/WSO_Global_Stroke_Fact_Sheet.pdf
10 million new diagnoses of dementia: https://www.who.int/news-room/fact-sheets/detail/dementia
5 million new diagnoses of epilepsy: https://www.who.int/news-room/fact-sheets/detail/epilepsy
2.5 million cases of meningitis: https://www.path.org/articles/toward-world-without-meningitis/
2 million new brain tumors: https://academic.oup.com/noa/article/3/1/vdaa178/6043315
1.5 million cases of encephalitis: https://www.sciencedirect.com/science/article/pii/S0163445322002110

That's each year, and that's only major conditions which directly affect the brain. Add in things like cardiac arrest, heart disease, sepsis, shock, diabetes, vascular injury, hemorrhage, poisoning, smoke inhalation, etc., all of which can cause secondary brain injury, and there are literally hundreds of millions of people every single year who suffer some form of insult to their brain which can lead to memory problems.

Do you want to trust all your coins to those odds? I know I don't.