Why? I am a bit surprised at the question, but here goes;
1. Pit out and blend lines mark areas of the track where there is a very really possibility of two cars being in very close proximity at drastically different speeds.
2. Cars in pit out do not yet have the handling of cars on a hot track ( cold tires and brakes ).
3. The physical lines give us an opportunity to remove ambiguity from judgements. In so many cases calls need to be made as to whether something is inside or outside of a space that we arbitrarily define like 3/4 car width, nose to the door, ETC ETC. Distinct lines allow us to make the calls and to provide absolute determination that a crossing was violated or not.
4. Cars in pit out rightfully think that they are outside of the racing surface and protected from cars at full speed. When I say protected I mean protected from anyone deciding to race in their non racing space.
If we go back and take a look at the video of the gentleman that was DQ during one session of the champs, and envision a car being in the pit out lane as those moves were happening. I think it would scare the shit out of anyone coming down the pit out. Especially at Mazda with the blind crest of the hill and potentially not being able to see if anyone is in the pit out lane. Just too much going on with no good reason that I can think of to allow encroachment. I think the better question than why we shouldn't allow racing in the pit out would be; for what possible reason should we allow racing in the pit out?