Why do most Lemmy users make posts with images hosted on external sites (not on the Lemmy server)?

Convenience of posting or not to burden their personal instance hoster?

That's why I do it. It's an easy way to support your instance.
- If something is important to me or feel like it could be of significance I will go to archive.org and archive it.
  Else it's not important. If it's just text, I will link it in my obsidian KB and copy the picture from the source if it's that important.
It is not difficult to open a link with a picture if the user uses Lemmy through a browser. However, when you use the Lemmy app, it becomes inconvenient to constantly open pictures through the browser.
- Using Sync so I can't relate to the picture opening in browser.
  Sync is very good with handling that specific part.
  
  Besides that my point still stands: Less storage needs and being a burden on the ressources of the lemmy instance provider.
- use an app that embeds the image / gif into the post.
  
  like sync, and most likely other apps too.
  
  some hostings are still not supported, but the main ones are.

From my instance sidebar:

To save on server-storage, please consider using third-party providers like imgur if you want to post images, thanks!

And also the recent issue mentioned by @dandroid@dandroid.app, a few lemmy instances wiped out most user-uploaded images as a result, and others no longer allow image uploads at all

Well since the CSAM issues, I shut down the image hosting part of my instance, and it will stay down until lemmy comes out with a way to disable image caching from other instances. I can't speak for everyone, but externally hosted images are my only option for now.

Because they don't want me (or many others) to see the image, presumably.

Because it doesn't support the latest formats.