This was a design choice by microsoft to maximize protection in server 2016.
Windows server 2016 disable antivirus.
Unfortunately microsoft decided to remove that api option in windows server 2016 so even if you install another antivirus product windows defender will still be enabled.
When the policy opens for editing click enabled.
Microsoft has also removed api s for other antivirus software to cleanly stop and disable windows defender av.
Due to changes from microsoft in the server 2016 architecture windows defender antivirus will not disable itself when other antivirus products are detected.
The user interface gui is installed by default on some skus but is not required because you can use powershell or other methods to manage microsoft defender antivirus.
By default microsoft defender antivirus is installed and functional on windows server 2016 and windows server 2019.
When remove features page opens.
Enable the user interface on windows server 2016 or 2019.
The next step is where you disable windows defender in server 2016.
On the details pane of windows defender antivirus node locate and double click turn off windows defender antivirus policy.
In server 2016 if you want to use another another antivirus program to protect your server the windows defender will not disable itself as it happens in window 10 in order to maximize the protection in server 2016.