It makes sense for an industry to grow if the consumer base is growing. E.g. maybe all these home-cleaning startups make sense because a whole lot more people are now getting their homes professionally cleaned than were a few years ago.

But how can advertising "consumption" increase in the same way? Either people buy more stuff (unlikely, because people already spend all of their money on stuff), or advertisers spend a bigger proportion of their revenue on advertising (which also seems unlikely, because their business models haven't changed). It seems more likely that most successful advertising companies are making their money by cannibalizing the rest of the industry. In which case, why is the industry growing?