In larger towns and cities every 3 to 5 average congs can easily be made into 2 or 3 larger congs.
A move like this might backfire on them and they know it. There is a reason why congregations are kept to a certain size. It is so elders can assess each publishers "spirituality" (aka guilt tripping the publishers).
If congregations were say 2 to 3 hundred publishers and there were 5 elders the majority would slack off.
A congregation is just like a hive of bees. If the honey isn't taken away regularly and the hive isn't divided the bees don't make as much honey.