I would guess making their phones seem more secure because people get less malware. I still think it's stupid tho.

Dark patterns work, they have the data.

See, the field of UI/UX design is very concerned with how to make the actions a user wants easier, how to streamline common actions and clearly communicate what each item does. To that end they've studied how apps get used with user interaction data. You can track with statistics whether cartain actions get taken more or less often with each change, and it's very clear that the more numerous & obscure the steps are in between a user and a task, the fewer users will complete it.

Of course this doesn't tell you what a user wants, only what they do. To understand what they want you need to couple this process with user reports and complaints to see where the pain points are. The UI has to balance how many steps an action takes with how cluttered the interface is. Some actions must be prioritised.

However, a company doesn't need data to know what it wants users to do, and it's a very simple step to take all this data and understanding and flip it on its head, to stop users doing what they want, and on average it makes a difference. It might not stop you, but it might stop your grandparents, or Dave from accounting. That's the problem.

So the short answer is, they hope to reduce adoption of alternative app sources.

I know the EU is taking steps to make this sort of thing illegal, but it's difficult to prove. I also got a letter from a consumer advocacy board in my country warning about dark patterns, so it seems like attention is starting to build on this issue.