Security is the biggest bullshit excuse for anything. If security was needed then why are there vulnerable apps on the Play Store?
True reason is they are concerned people are side-loading 3rd party browsers and apps to bypass ads and premium stonewalls. All in the name maximising whatever small % of ad revenue.
Google is ending soon the "Wild West" era of Android app installation. The company is introducing mandatory developer verification and digital signatures for sideloaded apps to increase user security. Now, Google is offering more details about it, addressing some key questions and claiming that the sideloading practice is "absolutely not going away."