Why Is Fan Suction/Intake So Weak Compared to Exhaust?

[sophiecentaur]

In principle, the laws of physics are symmetric and time reversible. If the input to a fan is omnidirectional, there is no reason in naive principle why the output of a fan could not also be omnidirectional.

The thread title is too vague for a proper answer.
I imagine the discussions above assume knowledge of the context of jet engine technology.
Context could make a huge difference but the simplest model would be appropriate here because the OP leaves the subject wide open. I had to start with a small inlet aperture and possibly the same size of aperture out. At the input there cannot be more than a pressure difference of 1Bar in still air. More than a very small distance away from the aperture, air will flow from a hemisphere and there's nothing that can change the flow out there. With a large aperture you could perhaps tinker with the air flow in the plane of the aperture but you'd still have the basic 1Bar limit. Is there any way that the pressure profile across the inlet could be changed to make it directional? That's simple 'suck' dealt with.

There can't be any basic limit to the amount of Momentum that the fan / pump could transfer to air in the centre of the jet. So you could say that the possible Blow has no basic limit, given the right design. There has to be a limit to the volume available (mass flow) but squeezing the shape of the jet would allow more 'blow' than the 'suck'.

You can't provide supersonic flow at the inlet ( without starting with fast moving air) but you could, over part of the outlet.