The reason is that soft casing is way more important than knobs when it comes to rolling resistance and MTB tires can be made soft because all the additional rubber coming with width while gravel tires are usually harder and thus slower (and less grippy and less comfotable). Notable exception is Continental Terra Speed but it's still not as good as Race King and way more puncture prone.
The problem with most current gravel frames is that they don't fit wide and fast MTB tires so you are stuck with the worst of both worlds - slow, not comfortable and still not grippy/soft enough to go fast off-road unless it's a very well maintained road.
