Google is doing what Apple does and implementing Gatekeeper-like signature checks to ensure only apps by Google-approved developers can run on Android.
Microsoft does something similar with Windows Defender: you need to buy a developer certificate that can be revoked at any time if you want to distribute your app and have users be able to run it.
We're at a point where we need permission from trillion dollar companies to run the apps we want on the hardware we own.
Clarifying: you CAN run an unsigned app just fine on Windows. A lot of freeware/"indie" (for lack of a better term for small software) programs run just fine, the only thing that happens is the user recieves a warning they have to press "Yes" on (which 95% of people do, because That's The Windows UX[patent pending]).
https://cdn.advancedinstaller.com/img/prevent-smartscreen-fr...
In order to run, you have to click on "More info", and then a second "Run anyway" button appears.
There's way more than 5% of the Windows userbase that gets confused and can't get past this warning.
I also haven't seen any specifics on how that system is supposed to work, but have seen a lot of speculation and (perhaps not unwarranted) fearmongering.