Yup ghosthouse, Geoff already stated the case---it’s a matter of friction. The plastic of the caster cups isn’t smooth (on the microscopic level of the vibrations we are talking about) or hard enough to provide a friction-free surface for the ball bearing to be free to move across.
But I agree with you, Diament’s wooden Easter egg cup suggestion is worse than your plastic caster cup one, wood being way too soft and textured to work well as a bearing cup. Barry proposed it to anyone wanting to try the roller bearing idea on the cheap, before spending any money on real ones. A poster on his site is the one who suggested concave drawer handles/pulls, the best idea for ultra-cheap bowls. The good news is that Ingress offers a set of three top/bottom cups for so cheap that the even cheaper alternatives aren’t necessary.
IMO, what Geoff has long suggested for isolation---the combination of roller bearings for lateral/rotational, and springs for vertical, provides a whole lot of isolation for not that much dough. To get more, one has to move up to the Townshend Seismic products (around $350 and up), and then up to the microscope isolation platforms by MinusK, Newport, and others. That’s some serious money, over $2k!