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.

*Panasonic Toughbook

(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.)

It's worth noting that by the time you make a rugged laptop it's actually bulkier than a small laptop bag with a handle.