See I always wonder that.. but if you were going to have special rooms like sickbay in some kind of bubble of non shaking (like they are always upright even if the ship does a 360) wouldn't the bridge be one of those special rooms?
I was thinking more in terms of random overload. That is, every
room is a "bubble of non shaking", but any room has equal odds of having the IDF system fail. Except that enemies so often like to target the bridge, giving it greater odds...
Also, depending on the deeper mysteries of IDF theory and practice, being on the outer hull might make a room more prone to shaking, and the deepest core of the saucer is the location least likely to shake.
The other possibility is that tabletops have their own built-in safety measures: extra gravitic pull, cheap localized damping fields, whatnot. Something that can hold on to a vase, a book or a beaker full of death, but won't cope with 160-pound Lieutenants.