Heres why: I haven't seen one yet that doesn't read "99" while you're stomping on the brakes. Yes, while coasting you're theoretically the most efficient. But braking is the most inefficient thing you can do while driving.
Think about it, you've already attained forward motion and now you're trying to stop it. That is the complete opposite of efficiency, yet your display is giving you atta boys for it. On a vehicle with regenerative braking, it's even worse because you're getting green bars or bonus points or whatever for braking. Even though you may be charging your batteries, you're still wasting the fuel that got you to speed whenever you're braking.
I think displays that train you to hyper-mile need to account for braking as a Bad Thing. Turn red, go to "0 MPG" or whatever. Has anyone ever experienced a car that does this?