Why does Google always ask me to choose an account when I add Google Drive?
Short answer: On purpose: Storage Studio asks Google to show its account chooser rather than silently using whoever is signed in to your browser, so you always add the account you mean and second accounts add cleanly.
Storage Studio asks Google to show its "Choose an account" screen on purpose, instead of quietly using whichever account your browser is already signed in to. That way you always add the account you mean, and adding a second Google account works cleanly.
If the account you want isn't listed, choose Use another account and sign in.