Recent ransomware events often include data theft in addition to just encrypting data. The Encrypting File System on Windows Pro/Enterprise editions encrypts individual files using a private key stored alongside certificates for that user account. The file is therefore unreadable by anyone except the logged-in user account. When you copy the file to a flash drive, it remains encrypted. The only way to read the file on another computer is to also copy the certificate + private key.

Therefore, if ransomware simply uploads your files, wouldn't they remain encrypted with EFS and thus unreadable by attackers?

This wouldn't prevent the ransomware from encrypting your files, but would it prevent threats of public leaks/selling of stolen data?

Edit: Assuming Windows 10, and assuming ransomware runs on the user account that can access the EFS-encrypted files.