It is Google's doing. I use a file manager app called total commander and the developer of the app mentioned that Google has asked him to stop allowing his app to install apks. So Google is probably moving in that direction where you can't install apks that aren't from the play store. It's really stupid why they would do this cause their own file manager app can install apks ( I'm on Android 12 still)! I haven't upgraded to Android 13 yet so I'm not sure if they have completely stopped allowing manual apk installations. And I don't think I will be of they are going in this direction...