Porting the engine is what a port is. I think the main problem is that there are several different versions of the source engine in existence ("hl2" source engine (not used for hl2 anymore), Orange Box, L4D2, Alien Swarm)
porting a game is far more than just porting the engine. Peopel keep underestimating the work that goes into making a game linux compatible.
Besides, Half-Life 2 is already available for Mac so they have converted all the functions to be unix compatible.