The width is for safe travel by people in wheelchairs. I've been in situations where a mere inch too narrow meant we had to go back to wherever we could cross, or drive/walk in the road, or even give up on that street entirely. Could you possibly fit the pavers if you took it all the way to the edge? Would it be safe for that stretch of roadway? The people who laid the sidewalk presumably measured and decided not. Now, could they have paved the straight AND the curve? Probably yes, but at greater expense. And we'd miss out on this nice desire path.