Software can be "shipped" to users in ways you may not anticipate. Even a lowly terminal emulator might find its way into a POS system, factory line, etc. (These interfaces are often shockingly primitive).

And even if not, developers might use internal code search, find what they want, and then copy and paste. The pushback on AGPL code (and GPL code even) comes from the difficulty of establishing internal policies to keep the code segregated. Much easier to have simple-to-undersatnd policies enforced at the boundaries, e.g. "no AGPL, period", instead of "AGPL code is OK for software that won't interface with our systems, as determined by either biased engineers or technically-shaky lawyers."