Because upgrades are susceptible to change the proprieties of the production queue etc. So it is only supported to upgrade an entity that has an empty queue be right now, else bad things could happen. I project to make a mitigation for this and allow for upgrades to be queued regardless but not sure how this will go.

Isn't all other meat just a giant steak when carried?

Why can't I queue the upgrade?

Size comparisson:    

well I don't think they have a long building time anyways