I'm not sure if your question is rhetorical or not. But if you really are wondering, it's because if we click the link, chances are that we read that first paragraph of text paywalled articles usually show. That makes us interested/curious and we might pay to view it. We also feel more invested in the content after that first click.

If we only have the title in the HN feed chances are lower that we will buy the content instead of simply scrolling by. Since the latter that takes less effort.

We humans don't always make rational choices, so nudging us in the direction of paying for content – if we want to support payed content – will of course increase the amount we spend on content.