This because Marx envisioned the transition happening after capitalism hit a crisis, where upon the workers would take over and continue to operate the factories within the existing supply network (more coops, less central planning).
In the USSR however it seemed one got more akin to the old Ford factory, where raw iron and coal came in at one end, and finished Model Ts rolled out the other.
Thus there were no independent sub-suppliers of parts, everything was made internally as needed, and nothing more.