I think because of the instant gratification a summary gives. It's a bit like how spoilers work, no need to watch that movie anymore of you know all the plot twists. No matter how beautiful the camera work is 🤷‍♂️

I (unfortunately) work in an industry where companies used to live or die by the amount of website traffics they received. 5 years ago the goal was to write the best content for your users, abiding by the EEAT principle - Experience, Expertise, Authoritativeness, Trustworthiness. Basically good advice given by proven experts. Since the rise of LLMs it’s like we don’t even consider human users anymore. All content is geared towards having your brand show up in AI search. The content itself is all AI generated drivel, but no one cares about that as long as ChatGPT mentions your name when asked about a related topic.

Edit: to answer your question more directly, websites exist for users. Your website only gets users if it appears in search. It used to be that higher quality content was the way to do that, but now that search is largely driven by gen AI, the quality of the content is less important than the specific structure and “freshness” (i.e. how recently it was published) that AI platforms prefer.