Because the tips are not open when needed (evenings) because you have to carry the bags up a flight of steps to get into the bins, because the tips are too small and overcrowded at weekends, because the average larger vehicle is too high to get in, so you have to carry the stuff even further, etc. etc.
It has always seemed to me that the council don't want to help people get rid of rubbish.
ps I have just got back from taking a trailer load to the dump, and everything had to go in the right bins, it really was a pain in the arse !! (but personally I shall still go to the dump rather than fly tip.)
mrcheerful