I think for the most part it comes down to this: People can't always afford to financially justify buying all of it.
Some hypothetical examples:
Hypothetically speaking, say I were to be an avid gamer (single player), aged 15-17... say 10 games come out each year, at an average price of R350 a game.... that would be R3500 a year spent on games that one could possibly play for a week each. A lot of people can't justify spending that sort of money.
Taking series into example: The majority of us have DSTV. Prison Break (for example), airs once a week, at a bad time for us.
What do we do? We download the Prison Break series. Technically speaking we already pay to watch it, as it airs on DSTV. The time that it airs is difficult for us to stick to, so we watch the downloaded shows.
Music: I suppose that this once again comes down to a financial justification. The average album has 10 tracks, costs R160. That's R16 per track, half of which we might not like. There might be 10 albums released a year that you would like one or more songs from. That would be R1600 per year that you'd end up spending.
Movies.... probably the same justification, although I'm less inclined to agree.