It's the principle.

I get that you think it's not a major inconvenience, but if I now throw yet another (pointless) popup for you each time you install an app, are you OK with it?

When I install something on my PC (Linux), I never get such a prompt. If any Linux distribution started giving a prompt on each install, power users will stop using it.

If I installed apps on linux at the frequency I installed apps on my phone, which is a few apps per year, I wouldn't care at all. That being said, I get fewer interruptions installing apps on my phone than I do on most linux CLIs, since on my phone I just press install and it installs, no question. The real problem is first-launch pop-ups/notifications/settings, and linux apps can have as many first launch popups as my phone does.