Easy, the ones displaying the ads have been paid to do so. Magazines of yesteryear have been replaced by websites, TV still follows the same model, but has probably been outpaced by the scale of ads online. Billboards are probably the only one unaffected, and the people who lease the space to erect the billboard charge their customers for the right to have their ad displayed, just the same as any of the others above. Want your ad in my magazine, on my TV channel, webpage, Youtube? Pay for the privilege.