Why? They should. IF they are actually squawking, and your installation is not masked by the airframe etc.
Exactly.
This isn't exactly an amature installation, and the condition in which I see traffic missed occur when I'm looking right at the traffic and it's the only traffic around. It happens, and I've seen it on a fairly wide variety of equipment, installations, and circumstances. Even the costliest and best installations still miss traffic. Go figure.
Point is that the law is see and avoid. This applies in IFR flight every bit as much as VFR. If you can possibly see it, you're responsible to be looking for it.
Use all the other equipment and gimmicks you like (and if they're available, you should)...but there's no substitute for the Mark 1 eyeball. See and avoid like your life depends on it.
Because it does.