Verified answer

because not every photographer likes it when someone uses his/hers photo without asking for a permission. it's basically to prevent image theft. in fact even if a photographer puts the "all sizes" button on photos, this doesn't mean that you can use the photo for whatever you want.

i think it's not really a big deal if you copy a photo and use it as a wallpaper, but someone can copy a picture and use it for various purposes, i.e. put the picture on photo calender and then earn money. in that case it's really a theft.

if you really like some photo and want to put it on your screen, just message the photographer. usually they agree to share photos with you, well at least it always worked for me.

I believe the user who submitted the photo has the option of disabling this option on Flickr.

Sometimes photos are not uploaded in such a large size also but I think you're running into a problem with the flickr user's preferences.

Also, I think you should still be able to right click the photo and choose set as Desktop Background in your browser but on small images this may not produce the desired effect (small image with large border or scaled image will appear distorted).

Hope that helps!