I have a question after reading the discussion here: Ransomware encryption keys
So I understand the process of WannaCry encryption now, thanks to the accepted answer given by the community on the security.stackexchange page linked above. However, what I do not understand is why generate a different AES key to encrypt each file on victim's computer? The attacker could generate just one AES key and encrypt all files using that and then protect that key using the mechanism discussed in the page linked.
Why use different AES keys? Does it have any cryptographic, strategic, or operational advantage at all for the attacker? Or is it out of pure paranoia?