Why is that assumed especially in the early days? My toddler has generalized intelligence but is no closer to spreading to another vessel than he is to teleporting to the playground. The first AGIs are going to run on massive clusters of very expensive hardware, its a large logical leap to assume there will be equivalent computing power it can leak to and that it also comes online with the smarts to reengineer itself to fit in that domain. You can have AGI that starts dumber than an an average human.