First time I started Windows Sandbox was to test install some software program I didn't trust and want to see if Windows Defender would detect virus, adware, etc before installing it for real. Turns out that Windows Defender is disabled in Windows Sandbox and you get an access denied when you try to enable it.

Installing it in the sandbox first does not help you determine if a script or software is malicious and that removes my primary use case for this feature. I sometimes wonder what exactly the Windows Sandbox if for if anti-virus wont run inside of it.