Where's sftp? s3? smb?
I would highly suspect this is because they don't want to handle encryption and decryption on their servers where ever they can. Yes they need encryption for emails they receive but for every thing else they want to have as end to end protected as they can. Which means client side encryption and decryption for almost every thing. That is the big feature that a lot of people are buying into from them.
All these protocols don't feal with encryption on the client at all. Which means if they supported them they would need to handle encryption and decryption on the server side. Which I doubt they want to do and likely work hard to avoid at all.