To them the player character is just as good as a belt for transporting materials.
The player character actually does have an extremely high capacity so if you are constantly running between chests your factory will still be reasonably high throughput.
What are you talking about? Maybe at the beginning, yes. Everyone carries stuff around when you have 2 assemblers, but once you start building your bus, the throughput of the player is orders of magnitude worse than using belts (or, if you're into that, bots)
For the purposes of launching a rocket the player character's inventory is vast.