It's a philosophy as to what you put on each. I reserver the Calendar for appointments, stuff that has to be at a certain day and time. I also use the Calendar to mark certain dates as significant in some way, as long as they don't require an action. Everything that is not hardwired into a specific time I put in reminders.
By the way, they are linked at iCloud.com where you can see and edit the Reminder tasks in the Calendar app.
If you set up notifications right, you will see both calendar events and tasks for the current day in the Notification Center.
So, I get reminders for the day at the Notification Center, both hard schedules from the Calendar, and soft reminders from Reminders.
For planing I check the Calendar for what must happen that day.
After that I look at Reminders to see what else I can do, or for specific lists I'v accumulated (like the shopping list) when the situation applies.
I suppose it's a mindset I got from reading David Allen's book Getting Things Done (yes, the origin of the GTD thing). I even read the sequel. Not that I actually bother with most of it these days. Life is lazy right now.
Though I can understand your desire to see everything at once in one app. There are several third party apps that will do that for you. I believe Pocket Informant is popular, and integrates with Calendar. Maybe they'll get around to integrating it with Reminders too, if that is possible.