Why does the Carry Cot on the stroller, and Oscar/Otto seat in full recline, no lie completely flat?

The Carry Cot - and Oscar or Otto seat at lie back mode - is designed to sit on a slight angle (3-4 degrees for the Carry Cot and 10 degrees for the stroller seat) when installed on the stroller.  This is a specific safety feature for newborn babies. Unlike at home, walking in the outdoors is never a flat surface and because young baby's can't always control their reflux, this feature helps to keep baby's head above their feet as much as possible, reducing the chance of vomit, or other bodily fluids getting stuck in their airways.  

For the Carry Cot, this also reduces the likelihood of baby sliding head first when walking downhill.

In addition, for some countries this feature is a compulsory safety requirement.