It's because size 0 units cannot be a target of a command. They cannot be hit by attacks, they cannot be selected, cannot be repaired, etc.
As a workaround, if you really need a size 0 unit, bearing in mind yours, which will not kill itself off, can completely derail the AI, give the unit's time attribute and the build skill's speed attribute sufficiently high values so that it's built in the first tick.