There were numerous experiments with how people memorize things and excluding elements from memory is normal, so translating this to bitcoin seed phrases, if you happen to have a thing like:
cabbage grape jealous banana
your brain will try to associate the three words that are vegetables and fruits which would lead overtime with losing the 3rd word in the sequence.
I bet you have memorized some lyrics that you learnt at school and you still remember them exactly. Well, at least I believe. I would say that memorizing is not a bad method if person tries to memorize it every day and still repeats it even after being confident. By repeating the same words every day for months and more, you will remember it so well, you will be surprised. But I think all methods should be individual, I don't force anyone to follow this one cause I don't know their cognitive abilities but I just think it's not as bad as people say.
Lyrics! Lyrics have logic and are easy to imagine or to capture a feeling:
I, I go my own way
I swim against the stream
Forever I will fight the pοwers that be
The eagle flies alone
logic and easy to remember replayed 10 times!
Now when you try :
foot ball wolf pledge obvious arch penalty
Your brain will try to first relate the two images, foot and ball, so the image of you kicking a ball, wolf comes easily, you kinking a ball at a wolf , easy to remember, then you get pledge and obvious and arch at which you will realize as you read your brain can't make a logical picture of it, easy to test as you will automatically try to delete the first picture in your mind to come up with a new one based on the next words only to relate again penalty to foot and ball.

Just pick any seed phrase and you will see how easily your brain imagines the first 2-5 words only to struggle involuntarily as the list goes on to create some logic.
This is not specific about bitcoin or a seed phrase, this is how our brain works. we're just bad at creating and memorizing random data, tests of this kind have been done for decades, and the conclusion is always the same, we're not built for it!