Because the others are made by companies trying to maximise their profits at their customers expense, since they recognise that most people are ignorant and intend to take advantage of that fact as much as possible.
These DVD's may work .. but a few months down the track deteriorate anyway. The result is that you have to back it up again anyway a few months down the track.
Buy good stuff, it saves you time and money.
My free-time is valuable.
Please put a blank verbatim DVDR disc into your drive and run the nero infotool & post the resultant log between # code # tags.