There's some rather dubious archaeological statements in this piece (but then again, tis from the Daily Shame) but I have to ask - how the hell could they not have seen it??????
The stone is...err, was... on the opposite side of the road to a large parking area and you would have to be a very incompetent driver to manage to hit it.
It is at the centre of a grassy car parking area and lower than a car in height - some people simply aren't gonna notice it as they manoeuvre their vehicles. I watched someone prang their wheel arch on it in July this year. They didn't damage the stone - just their Nissan!