It should be possible to repair nets with tying equipment. I guess the reason why this isn't possible yet is just the fear that fishing might become to powerful, but this will not happen as I can explain.
Hunting, agriculture and fishing are the most important ways of acquiring food in the game, and while it is possible to play it mainly as a farmer or hunter, it is not possible to play it as a "real" longterm fisherman. For large-scale agriculture you can prepare more soil and plant more crops. For large-scale hunting you can place more traps. For large-scale fishing you can place more nets, but... the nets wear out. You are strongly dependent from the villages und have to stay near them, but at some point just all the nets will be consumed and you cannot keep up your playing style.
Even dairy farming is better than fishing this way.
It would be arguably okay if it took days to repair a single net, since fishing itself is a quite good source of food.