It's not really anything to do with monitor refresh rates, it's just to do with the internal physics engine. In Quake, the physics weren't independent of the display output, so the FPS values where you moved quickest were something like 45, 72 and 125. Machines weren't fast enough to reach 125 back in the day, so everyone capped at 72.

Perhaps the same physics/fps values effected half life - half life was a very 'platformer' style game in many sections, so perhaps they wanted to make sure that the physics felt the same for the majority of the player base, locking it to 72.