The recess that holds the glass has four square corners, and that is the reason it is cut by hand.
A rebate like this could be run around the frame with a powered cutter in minutes. But every powered cutter is round, so it leaves a rounded corner - and glass is cut square. Fit square glass into a recess with rounded corners and it sits proud, rocking on four small radii, with visible gaps at each corner. The usual industry answer is to round the glass corners to match, which looks like exactly what it is: a compromise made for the machine's benefit.
Ours are chopped square with a chisel instead. The rebate is cut around the frame, then each corner is pared out by hand to a true right angle, tested against a square, and pared again.
Then the clearance. Glass will not be forced - it does not compress, and a panel wedged tight into timber that expands in the monsoon is a panel that cracks. So the recess is cut a little larger than the glass all round, and the gap is packed with small soft setting blocks that hold the panel centred while leaving it free to sit rather than be gripped.