Those users are Apple's target market, you are not.

The main reason people buy Apple devices today is that the software ecosystem is consistent and predictable. This isn't silly, this is a core value proposition.

Meanwhile, millions of Android devices have shipped with with literal malware from the factory. I know because I used to develop custom ROMs to remove the malware that my own carrier installed on my phone.

I doubt Apple is primarily concerned about losing money to side-loading. The vast majority of users purchase apps from the default store on their phone.