Does there need to be? It wouldn't be precise anyway until everyone was sat down and their bags stowed. By which time it would be too late to do anything about it. And if you were getting into that level of detail it would be constantly changing anyway once people start moving around the cabin.

But if you know in advance that the flight is full. And two thirds of passengers will be sat behind the centre line of the aircraft. And the average weight of each piece of the local walking cargo is already significantly heavier than the global average. And if those passengers also have excess cabin baggage stowed near to them it further increases the weight behind the mid line. And the aircraft is already approaching the limits of its range due to the actual distance, total weight on board, and bad weather or diversions en route. And all of that extra total weight on board requires a lot of extra fuel, the weight of which also requires extra fuel, putting further pressure on range. Wouldn't you want to tip the balance (literally) a little more in your favour by redistributing some of that excess loading from the rear to the front, improving the aircraft's handling, performance and ability to reach its destination without stopping?

But I get it. Individuals don't want to check their excess cabin baggage, or think it shouldn't apply to them. The aircraft operators think differently.