This is because our brain can easily recognize patterns. The way the seedphrase is memorized and practiced, our brain conceive it as a pattern so when we try to make it in random, the pattern in our brain is broken thus we have hard time remembering the next word.
I agree, if our brain develop a pattern on the seed pharse it is much easier to remember because it all needs is the initial string and the next will automatically follow. But obviously people brain has limit. Memorizing is good for those who have high retentivity but bad to those who are not. There are also cases where the brains deteriorates and start forgetting things. This is disastrous to those who rely on their memories for important things. So to be sure about not forgetting things, a more reliable back up is needed.