Why...
I suppose, in part, because cafe style is big these days.
The value goes up as demand does- and there really aren't that many.
If you like the style, GBs are gorgeous.
They are a 500 single. Singles just have a different appeal to many people and really can't be compared to a 4. These bikes are incredibly light and facile.
The downside is, they are purpose-built and can get crampy on long rides. They are smaller,. overall, than you would think just looking at pictures.
Another downside is they are hell to change oil. Messy. Oil all over the place. Hard to check oil level.
Great in the mountains and around town.