don't know why it is this way now, when the gun was first added to the game you could look around normally like any other manned gun does.
Am I allowed to take a guess?
The 17 is the only field gun with a proper gunsight, it's using the same "gunner" position as tanks, not as other field guns.
Now when the gun was introduced, it did not matter, as at that time with the old GV system tank gunners could look around (inside the turrets) as well.
The new GV system abolished the ability of gunners to do that, slaving the view to the barrel. The 17lbr gun fell victim to that, it's a kind of 'collateral damage' of the new GV system.