Real Particles Take Up Space
The ideal gas law assumes particles are infinitely small points. But real molecules have size!
Start by adding a lot more particles with the create tool.
Now shrink the box using the slider below.
At high density, the particles are jammed together. They can't overlap; each one occupies space that is unavailable to the others.
The pressure is much higher than the ideal gas law would predict. The effective volume available to each particle is less than V because the other particles are in the way.
This is the excluded volume effect. The correction parameter b in the van der Waals equation accounts for this. The actual free volume is V - nb, where n is the number of particles.
At low density (big box), this correction barely matters. But cram the particles together and it dominates!