As for why we use Google services. It's because most professionals do use Google Calendar. Some don't, we understand. We might serve them in the future but we have to start somewhere.
We advertise the desktop app (instead of the Web app) because Routine is intended to be used through a desktop-wide keyboard shortcut so you can capture thoughts and tasks from anywhere. A Web app requires that you switch to your browser, then find the right tab and click it to then be able to interact with the Web app. We believe it is too slow for a productivity tool. But some people do not have a choice (Windows) this is why we've answered them.
If you do not trust our service, we understand. We don't force you to use it. It's obviously your right.
But just to make things clear (which we explain in the Privacy section), all tasks are stored in your Google Tasks account. All events are stored in your Google Calendar. And all your notes are in your Google Drive. This means we store no data on our servers (for now at least).