Mechanizing the attachment to the tab from the servo is probably the reason.
It's simpler to cascade servos than finger out how to make such a thing operate.
.
Or, it could be simply lack of confidence in the ability of a servo tab to perform at all the flight conditions a 3D plane encounters..
Lots of zero airspeed stuff, where having the suface fully deflected by the servo train can be depended on, rather than relying on low air pressure to move the surface the same amount.. or hold it there..
See a similar thread on boost tabs.
I've used boost tabs, but can't say they worked any better than no boost tab.