Big brands spend millions establishing a particular look, style, format. They don't want you to treat their sites as merely a set of APIs to scrape and customize based on your own style preferences. They want you to have a branded experience.
You’re absolutely right. But consider big brands make for a minor percentage of sites on the web. Also recall that all those big brands have standard profiles on social media and they share the very same layout as your local dog shelter. They have no problem with that.
They do have a problem with that. I don't see companies bigger than the dog shelter directing users to their Facebook page anymore. They all have unique looking websites.
It doesn't really matter what they want. Chat interfaces are doing this from the opposite direction, pulling the data down and explaining it to you, it's not a big leap for LLMs to turn their markdown responses into a slightly richer experience you can browse natively.