Goals are to be celebrated and, like most celebrations, they are steeped in ritual. With the goalkeeper beaten, it's important that the ball hits the net.
(If you're not sure that the ball even needs to hit the net, see the clip below and consider: what if Andy Gray had walked the ball over the line and picked it up, and ran back to the halfway line, ready to re-start the game? It would still have been a goal, Wolves would still have won the Cup, but would it have provoked the same riotous celebrations as him smashing the ball into the net?)
With the ball in the net, the ritual of celebrating a goal insists the game pauses for a few seconds for two reasons:
The fans can celebrate the goal, and;
Because nearly every goal scored turns a game in a new direction - even seemingly meaningless 'consolation' goals like Newcastle's first in the recent 4-4 draw with Arsenal can be a turning point - it is a timely pause while the new equilibrium of the match is realised.
The best way for the game to pause at this vital time is for the ball to stay in the net (or at least behind the beaten goalkeeper).
Dictionary.com defines goal as "the terminal point in a race" and net as "anything to catch or ensnare," and for 100 years after goalnets were first erected in 1892, the natural shape of the A-frame goal helped the net ensnare the ball and terminate the action.
As previously posted, the free-hanging box goalnet is not a shape sympathetic to the ritual of celebrating a goal.
This won't stop goals being scored, and goals being celebrated. But the modern box goalnet's inability to retain the ball means the modern game is giving up another of the rituals that have served it since the beginning of time.