but I don't understand why we are not allowed to actively crossfeed (switching off the pumps on the 'good' tank).
1.....Remember Air Transat.....
The simple answer is because the QRH tells you not to apply the FUEL IMBALANCE procedure.
It can take well over 30 minutes to confirm a wing leak - over Europe you should be on the ground by then.
Other factors... the QRH procedure only says "a leak from the wing
may be suspected" (my bolding) - you do not know exactly where the leak is as 3holelover pointed out. If you choose to go against the QRH - which you are entitled to do if your course of action is safer....... not cheaper, safer .. just make sure you are right!