For as long as kiting existed, we all wanted what you are asking for.
My guess is: how would the company make it fair to all kiters, and put the exact amount, same shape, of material in the bag? Someone will always bitch about something. So, they just avoid the issue and don't put extra kite fabric in each bag.
Also, maybe it has to something to do with the import taxes, weight, etc.
Regarding the fix kit, there is kitefix, which was supposed to be in each Eclipse kite but was not. You want a kite as cheap as possible, well, extra stuff is not free, so....
What I think the companies should offer is spare material, but unless you fix your own kites, any kite repair shop has the material , so no need for us to deal with it.
If , where you are, there is no repair shop or material available, then it would be nice from the companies to offer it, maybe with proof of purchase, free of charge (+S+H).