This is actually required by german Law. Specifically § 623 BGB says:
Die Beendigung von Arbeitsverhältnissen durch Kündigung oder Auflösungsvertrag bedürfen zu ihrer Wirksamkeit der Schriftform; die elektronische Form ist ausgeschlossen.
Translation by me:
Resignations and ending-contracts to end an employment are only valid in written form; an electronic form is not possible.
The definition of the written form is in § 126 BGB. It basically says, that either a signature is needed or a notary has to vouch for it.
So if you are actually employed under german law you have to send them a signed letter. How that letter arrives at the employer is not specified. I would imagine most resignation letters to be personally handed in.