He can't afford to be losing overlords, when he is inevitably forced to retreat he will lose his overlords that are generating creep. Early game Zerg cannot afford to lose even a single overlord, larva and resources are in too high a demand to warrant wasting overlords on the chance that you may save a few hydras when you attempt to retreat after a forcefield doesn't totally box you in and force you into a disadvantageous engagement.
Additionally, the forcefielding of the ramp is not something you can counter, you have to have a queen at both hatcheries. You can't just choose where the extra larva will spawn what are you even talking about. You can't choose where your units are going to spawn, you are forced to generate from different sources due to the zerg macro mechanic and regroup at a rally.
MCs forcefields were quite literally not counterable.
XI Wiki


