To be honest, other than both of them allowing you to write markdown, they're not comparable.
Obsidian is the current favorite of the "make a second brain" crowd which is based the concept of a Zettelkasten [1]. There are thousands of plugins to customize Obsidian to turn it into whatever you want. It just so happens to use Markdown files to store your notes. It's a very powerful tool, but it's overkill for most people who want to write a few notes in Markdown.
Ghost isn't about wiki links, plugins, hypertext, Zettelkasten stuff.
It's just for writing, which I think is fine. Not everyone wants or needs all of whiz bang features of Obsidian or Notion or Microsoft Word.
Regarding that previewing isn't included; it's not a big deal in reality.
The Notes app that comes with macOS can import markdown files and render them. There a hundreds of apps, utilities, plugins, websites that enable a user to render a markdown file. For most people, that wouldn't be ideal; I get it.
(Aside: at a user group meeting, I saw a developer coding something in Vim with no syntax highlighting. I had never seen that before. He said he liked it better that way. Not everyone likes the same things.)
There's a great app from indie developer Brett Terpstra called Marked [1] that was created to preview markdown files. It has tons of features, all centered around previewing markdown files. I've been a satisfied customer of Marked for years.
We all use certain apps for certain things even if we have other options; sometimes it's for aesthetic reasons or we just like how a particular app "feels" when we use it.
If people enjoy using Ghostmd, that's great.