First because it's not an either-or situation. Marketing works by finding different channels and approaches. You could say "in a world with video, why would people pay for an ad on a bus stop" with the same logic.
Secondly because advertisement is about trust. For a while, the ads on TV where all the rage because, if it's on TV then it's a proper brand. If in the short term future, like I postulate, the web becomes unreadable SEO filled trash, and people access it via Chat-GPT like technologies, then there will be an element of trust between the bot and the user. And that trust can be exploited, more or less subtly.
