Because modern technology is so efficient that it creates an absolute endless firehose of resources and power which could do more or less anything, but the criteria that go into selecting who's going to be in charge of making decisions, and who keeps the bulk of the profit, are often a bunch of semi-random crap. So you wind up with people sitting on top of billions of dollars born of profits from donut-making, who have no real idea of what to do with the rest once they've kept most of it for themselves and their friends, and so they buy advertising with it.
Some advertising is quite effective; the leading edges of its technological development are targeted and refined to a terrifying degree. But there's also an incredible amount that's just noisy ineffective crap, basically an illustration of Sturgeon's law in the form of an expensive and irritating waste of energy for everyone involved in the process.