I remember years ago when practising engine failures in the sim, we would just pop open the cross-feed to make "one big tank", and keep flying with a nice symmetric fuel load.
When the manufacturer found out about it, they were aghast. The problem was that the simulated fuel pumps were all identical, producing identical outputs, and thus a matched fuel feed from each tank - so the procedure worked fine in the sim.
In the real world the manufacturing tolerance results in each pump having a slightly different output pressure. If you just open the cross-feed, then the side with the pumps providing the slightly higher pressure supply all of the fuel, overriding the output of the pumps on the other side - so you rapidly get a large fuel imbalance - and the side of the imbalance is unpredictable.