A previously discussed, the square shape of the box-net is unhelpful in retaining the ball once a goal has been scored.
Another factor to consider in ball retention is the modern bottom net support.
Nets used to be pegged into the ground. When a ball hit the net, the net would pull on the peg. In even the tautest of nets, there would be a little 'give' in the net, helping the net retain the ball.
However the bottom net support - basically a thin tube of steel that is fitted along the bottom of the back of the net -means the net is an entity entirely of itself. The only 'give' there could possibly be has to be within the net and support structure. As we have seen, the nets are often stretched as tightly as possible.
The bottom net support is practical. According to the manufacturer of the goals at Wembley, the bottom net support folds to enable ground maintenance so the net can be raised for cutting the grass, as in the photo to the left.
But the bottom net support is helping to kill the aesthetics of the ball hitting the back of the net.
The History of Goalnets says bring back the pegs. It's a small price to pay for beauty.