Because you reach roughly the same audience as the original article with your comment when it's published directly below it, whereas your social media post is read by your contacts only unless it's exceedingly successful.

Why would anyone pay $5 a month subscription to a YouTube channel that is already free?

The idea of selling things online was never about how to sell them to people who don't want them. You're clearly not the target market. It would be for people who are so fiercely loyal they don't think twice about how nice of a bonus it is to be able to be in the select audience that CAN comment. I don't know if people are this loyal to publications, but I've certainly felt it towards YouTube personalities that align with my thinking so strongly that I feel supporting them is promoting my values.

> Why would anyone pay $5 a month subscription to a YouTube channel that is already free?

To support the content producer. Maybe $5 is too high, but what if it was $2 or $3? I pay that to a few producers.