I, too, was astonished that this was not one of the STOCK aircraft included with the sim. But I got to thinking, and since the aircraft in FS are STRICTLY civilian, they obviously didn't include it becuase of its connections to military-type aircraft.
But I don't understand why it hasn't been modelled for FS since 98. But it sort of makes sense. After all, a very large amount of work goes into making an aircraft (believe me, I know

), and the X-1 was, when all is said and done, a rocket-plane. Its the same reason the X-15 hasn't been modelled since 2k2. Its just not within the long-term interests of the modelling community. You'd see a few instances of screenshots from it, but you can't seriously fly on and on in that aircraft. Its a one-use, specialized vehicle. There have been a few attempts at making these types of vehicles, but they all eventually fall by the wayside. The aircraft that last are the ones that are utilitarian; by their very nature, long-lasting aircraft.