It seems nobody answered your question yet. The reason is for privacy. As it is likely that most people using this forum own some Bitcoin posting an image would be a good way to harvest IP addresses if a proxy wasn't used.
The image proxy was implemented after someone put
tracking pixels in his post.
- it fails at showing the image 90% of the times
In my experience images load most of the time, but it can take a while. I have proposed to
set Alt-text by default for all embedded images, but you can already do that manual (click Shift-Reload to see it):