> the system would ask you to give it the privilege to install "unknown/unsafe app"
A tech savvy android owner wouldn't think twice about that. Anyone else (the vast majority of customers) would likely not continue, either because they're scared of something they don't understand (Fear Uncertainty and Doubt, aka "FUD"), or because they're annoyed and confused by having so many popups which they don't want to read.
Whatever the case, the UI/UX flow of getting your product onto a customer's device is critical. It's why companies pay OEMs so much money to preinstall their software onto their devices.
Optimizing the customer experience is one of the most important ways to beat the competition. It's an investment that provides value directly to the consumer.
But Google is restricting competitors from doing that. If you want to compete with Google, you need to force all your customers through the Google-designed funnel of scary warning and repeated technical popups. This limits competition, and harms consumers.