Why is it...

Correct! Lift is more a function of matching the cylinder head flow peak point to the camshaft (or what most cylinder heads and valve springs are set up for). But with flat tappets, lift will limit how much .050 duration can be implemented with a given amount of duration at .006 lift (or lift at lash clearance if it's a mechanical flat tappet) most cam grinders are willing to put onto a profile to keep the ramp rates on a prescribed (usually conservatively paced) schedule.