Because the ramp is designed for a hydraulic lifter so you need to give a little lash to get the engine up and running cold. I’ve set them as tight as .002 cold and they will start, but if you don’t let it build some heat the valves will grow before the heads and the lash closes up and it will hold the valve open.
Once the temp gets up a bit and things stabilize the lash opens back up a bit.