Because it will reduce mechanical grip
Bit of an old question, but it doesn't appear to have been answered accurately. I'll try to explain in layman's terms, which might mean some of the language might not be 100% accurate as per physics, but it helps to understand it!
It is possible to have a roll centre higher than the CoG by making the upper link in a double wishbone setup horizontal, and the lower wishbone angled downwards towards the outside of the car. The Triumph GT6 with rotoflex suspension has such a setup.
Note the very high roll centre
The reason this isn't done is that any roll centre above ground level reduces mechanical grip. Grip at the tyres is a function of the force (weight) they are subjected to. The more weight you have on a tyre, the better it grips (provided it doesn't exceed its coefficient of friction). You want weight transfer onto your tyres.
With a roll centre at ground level, the effective 'angle' of the force the tyres are subjected to is dead vertical. All of the weight transfer in a corner is acting down on the tyre, which is desirable. If you have a roll centre above ground level, some of that force will be taken up laterally, pushing sideways against the tyre sidewall and along the suspension linkages. It is this action that limits roll. However, this means that the tyre will be loaded sideways which is worse for grip. I believe an extreme version of this happens with the RC above the CoG.
However, there are compromises in design that make raising the RC desirable. One of those is if you have a camber curve that does not rise enough to meet your roll degrees, and a moderate amount of roll centre height helps keep the tyres negative. An ARB would also help this without side-loading the tyres, but comes with its own consequences and packaging requirements.
Another example of acceptable compromise in RC height is in high-downforce racecars like F1. If you look at F1 cars they have sky-high roll centres at the front which should result in awful mechanical grip. However, this will work to keep the car dead level in a corner which will allow uninterrupted aero. Any losses in mechanical grip can be offset by massive increases in downforce.