If I write it out by hand, it's most likely just for my eyes anyway, and I'd type it out on a computer if I'd want others to have a look at it. But even if I gave someone else my handwritten note, I think from context it would be pretty clear what the "squiggly lines" are supposed to be.
The same applies to the scripts used by e.g. Chinese, Arabian, and Korean speakers.
So naturally I am not a "fluent" writer of ξ and ζ, and since I virtually always write those characters in isolation instead of as part of words, it's a different mode of practice even when I do use them.
It's the same on the keyboard, by the way. My keyboard does not have ξ or ζ keys, it's all special.