It's also a key part of the marketing spiel -- selling people on the idea that they can get bitcoin-rich for free is a key part of getting people hooked on bitcoins. If I tell you "I've invented a new currency, want to buy some?" you'll tell me to sod off, but if I tell you "I've invented a new currency, you can mine it yourself as a background process" then you might be persuaded to give it a go. Pretty soon you've got some bitcoins of your own, and every incentive to start talking them up to everyone you meet.
"Unless you have a GPU farm, go buy bitcoins instead" and variations thereof.
Even right now, I doubt you can just grab a high end GPU and get cranking for a profit, unless you manage to steal electricity or steal access into a computer you don't own.