Users need to be able to edit the same files that ransomware encrypts, and differentiating between a legitimate user and a ransomware program is difficult.
No matter what the application does, it can't access the backups in such a system.
And if the gang get's admin rights on the box your backups are gone.
How is this program to know that a file edited by the virus to encrypt is legitimate or not when the edit is being made by a user that created and owns those files? The backups themselves can be contaminated months before the encryption and ransomware attack is sprung. Restoring from last week or last month's backup might still lead to your system being encrypted.
Additionally, as the other user pointed out the goal would be to gain access to the appropriate user with the level of permissions, such as an admin or root account, and use those credentials to carry out the attack.