Because bluing yourself is messy.

https://www.youtube.com/watch?v=9GYtgFdXCGE

Why not? You send the picture and ask to blur the monitor in plain text. It gives you back the picture with a blurred monitor.

That seems like a very easy way to do the job. What's the issue specifically?

The same reason today's inexperienced programmers depend totally on NextTailVibeJSFlare. It's all they know.

Honestly, if these image models still use diffusion with random seeds at their core, it might be actually more secure than blurring it yourself.