There are pins on the motherboard to install a Kickstart, albeit split over 2 chips. Some companies/users developed daughterboards that allow a regular Kickstart chip to be used.
Plus, disks are cheaper. And with OS evolving so quickly in those days (and the A1000's production window cut short by the introduction of the A500 and A2000) it probably wasn't cost effective to put Kickstart into silicon.
