Don't underestimate the cost of printing... Even in bulk, the cost of printing a decent manual is going to be several times more than just the box & disc.
You might also look at the resource use issue... I think it's a safe bet that maybe 10% of users actually RTFM and probably 10% of them will actually be inclined to print a hard copy... so you've just saved 99% of the world's trees.
Don't get me wrong, I do miss the good old days of glossy, spiral-bound manuals that you could beat someone to death with, but it's hard to justify that sort of thing both in terms of economics and environmental impact.