Have no idea why it's not an available feature for surfaces. Seems logical.
If you have a fairly simple surface, you could also sketch a line for your chamfer, surface extrude it, and trim away. Still a workaround.
There is nothing wrong with that solution Ross. Another method you could use is to fillet, delete the filleted surface(s). Surface loft to close.
Depending on the complexity of the geometry you could also sweep cut a circular section around the edge then surface loft.
As to why there's no chamfer for surfaces, I'm stumped, but I'm sure there's a good reason.
sketch fillet should be the easyest way of dealing with this problem..