If you don't want that choice you don't have to enable the feature. Installation from Unknown Sources in Android is off by default and requires a user to explic

If you don't want that choice you don't have to enable the feature. Installation from Unknown Sources in Android is off by default and requires a user to explicitly go into the settings and find the obscure toggle, which in recent times has proven to be a big enough deterrent that there don't appear to be any recent mass-scale attacks using this vector. On top of that Android and Windows both have fairly comprehensive parental controls, which can disable the entire option of installing non-approved software, so that point is moot. And the resources needed to make an app installer are not nearly as high as you make it out to be because iOS already has the mechanism to install signed .ipas. All that's needed in theory is a check to disable signing (which they already have implemented in MacOS) and to add a few pages to the Settings app, which surely shouldn't be an issue for a tech company of Apple's size. And if you argue that it might break some spaghetti code then maybe that should be fixed anyways and it's doing them a favour.