Why is the net flux in a cube surface area zero when the source charge is located outside?

Remember that for the flux calculation, there is a scalar product and the angle between the surface normal and the field at each point must be involved. The larger the angle, the smaller the flux.

If the surface is farther, the field is weaker but the angle is also lower. On the whole cube, the flux compensate each other.

If you placed any net charge outside the box (for a uniform electric field), the net flux through it is zero. Why? Because what we mean by a uniform electric field is a series of parallel field lines passing in and out of the box.

Uniform electric fields are generated in capacitor plates. The inverse square law would say that the field lines would diverge and you might think that this creates a non-zero flux, but at appropriate distances (in millimetres probably) that divergence would be minimum and we can imagine those field lines to be uniform as they are called.

What I mean by divergence of field lines is literally the spreading of theirs.

I think I understand your confusion. The field lines that you see in illustrations of flux do not get weaker over time, counterintuitively. The 1/r^2 dropoff of an electric field is not due to the lines getting weaker; it is due to the fact that the electric field is being spread over a wider area as it gets further from the source charge, and thus it is less dense. In other words, that 1/r^2 isn't a property of the field- it's a property of geometry. Each line you see entering a closed surface, like a box, is the same strength when entering as it is when exiting.

Hope that helps!

Just visualize the flux as an incompressible fluid flow (that's what the word means after all) and where the charge is a magical source of fluid. Now imagine a box-shaped region in the fluid. Because the fluid is incompressible the total amount of fluid in the box must always stay the same. If you have a charge (source) inside of then box then whatever fluid is appearing at the source must flow out of the box: there is a net flux. If the source is outside the box then whatever flows in through some sides of the box must flow out somewhere else. In this case the net flux is zero.