I wouldn't go quite as far as to say I think it's magic - It's just that different people put their own preference on a huge number of different factors when designing a circuit. These factors will include component price, if, and by how much, a component is overspecified; package type, lead time, minimum order quantities, ease of use, and so many more.
For me ease of use and package type are probably the most important factors - I'm going to prefer an octal sourcing driver in a through hole package over 8 individual surface mount FETs as I'll be hand soldering to a prototyping board - with overspecification and price being my perfectionist ideals. (Note that I'm not looking for the NXP LED drivers, as they are only available in surface packages, while the DM163 comes in a DIP flavour)
For the Colourduino/Rainbowduino people, they are going to have a different set of priorities, and they are going to focused on getting the BOM down as much as possible - smaller SMT components, automated assembly, reduced price (economies of scale), possibly concerned about guaranteeing future availability/lead times.
OTOH, 8-unit sourcing drivers are remarkably rare considering how useful they are for driving arrays. M54564FP is available from iTead at US$3.50 which, I agree, seems remarkably steep. I've looked for octal sourcing drivers before and didn't even find M54564FP.
And this is what I find so counter intuitive - the demand for these items exists even if it's only for the hobbyist market (worst case, it's only you and me looking for them) so why are they so uncommon and seemingly overpriced when you can find them? Maybe I should test the capitalist theories but buying 20 or so from the far east and putting them up on ebay to see how well they sell...