Because it doesn't. In a lot of airplanes it may, but in a 727 or a Piper/Cessna twin it does not (some aircraft can barely maintain altitude OEI). Any obstacles that do not penetrate the 200'/nm are not highlighted in a DP, so if you have that you are good to go. In the KC-10, we used to do 2-engine climbout TOLD for all of our takeoffs because we could - if our 2-engine data showed a minimum climb gradient, we were good to go on 3. Many aircraft don't have that kind of OEI performance and many operators don't do OEI climb data - we can't even calculate that if we want to for my current carrier. We calculate all-engine data and we see if we are good to go - but we have no idea about our performance in the event of OEI. That is why they have takeoff mins and Engine-Out DP routings - you follow the normal DP if you have normal performance - but if you lose an engine you go to the OEI procedure which for us is an alternate routing that is based on obstacle avoidance (it is flown ONLY if their is an engine(s) failure) or you fly see-and-avoid.