It’s not about open source, but free/libre software. Third-party developers being able to change the software is a side effect of who is in control of the software that keeps you alive.
Sure we can do certification and similliar, but we still can’t be sure what’s really inside. Also the is a problem of accessibility. What if the app that you need to be alive is made only available from Google Play Store for “security reasons”? Now you are tied to the will of Google if they want you to serve the app or approve account for you.
What free software does is basically makes a protection from many types of abuses that are implications of user not being practically able to do what developer can.