Unfortunately, left side cranks falling off of new bikes is a fairly common problem. I had that happen once in my shop while a customer was test riding. I took out my frustration on the newest guy in the shop. Handed him a torque wrench and told him to check every crank arm in the shop. Never happened again in my shop. Talking to other shop owners, I know that my experience isn't unusual. Nobody wants to talk about it because they aren't proud, but it's common.

Now I haven't toured every bike manufacturing plant in the world so I don't know how they all work, but in the one that I have toured, the bikes come down an assembly line and a guy tightened the cranks on both sides of the bike from drive right side using an ordinary ratchet wrench. I suspect they just don't get the left side cranks tight enough.