Weight. Packed size. Most people don't need a handle, so why waste volume and material on it?
I think the Toshiba Toughbook had a handle. I think many folks here, given their druthers, would design something that looked like a Toughbook, then become disenchanted with it for various completely foreseeable reasons, and go back to using their standard issue MBP.
(There were generations of these with handles, but at least in the early ones, the handle didn't dominate the dimensions - things like "removable drives encased in impact gel" did.)
I think the Toshiba Toughbook had a handle. I think many folks here, given their druthers, would design something that looked like a Toughbook, then become disenchanted with it for various completely foreseeable reasons, and go back to using their standard issue MBP.