It's just part and parcel of owning the domain. It's one thing to have the domain, but the next step is offering an active website so that people that actually put that domain in a browser can see it's a placeholder.
Why HTTP? Why not SSH? When I try to connect to it that way, I get no response. How will I know it's a placeholder?
This is not the point discussed here.
> Someone else could
When you own a domain, you can choose not to serve anything on HTTP/S, and still nobody can come and serve some other website on your domain, because they don’t control the DNS records.