Recording ~5 hours of television a night would have been a trivial cost for a network like NBC. Particularly compared to the licensing fees those hours would have had.
In general, the affiliate nature would add a wrinkle to the whole thing, but not an insurmountable one. If nothing else, they could have used the broadcast from the affiliates they owned.
You've quite a few barriers to getting that stuff online.
1. Sure, someone taped 6 hours onto a junk tape of TV from some channel to catch one show. But then they likely taped over that, again and again.
2. Tapes are bulky. VHS in general and junk tapes in particular would have been viewed by most people as low value junk that was tempting to disposed of. That's especially true during the decade or two before nostalgia and retro-cool starts making old junk more desirable.
3. Tapes degrade. Even if someone kept them, they might not be readable and/or gum up the VCR you're trying to use to read them.
4. VHS digitization equipment is also old. Apparently newer capture cards aren't very good compared to older ones, and there are specialized devices to fix signal errors (TBCs), allowing capture cards to actually work, that are becoming hard to find and expensive.
5. It takes a lot of time. VCRs play tapes at 1x speed. So if you want to digitize a 6 hour tape, it's going to take at least 6 hours.
Hook those up, record to DVD.
Rip DVD.
No need to fool with terrible capture cards they sucked back in the day and have not improved. The biggest problem I found with VHS is mold growth.
Edit: bonus with dvd recorders is that some have firewire ports so ripping portable video vamera tapes is automatic.
Broadcast-grade video tape cassettes were expensive even in bulk, and all tape requires climate-controlled archival storage since heat and especially humidity are quickly destructive to the adhesives that hold the magnetic layer to the substrate. (If you'd like more detail here, the term of art for this failure mode is "sticky-shed syndrome". While it's obviously more of a problem now than then, archival needs were understood at the time.)
Depending on format (Betacam SP or U-Matic), an open-ended commitment to preserving all programming would involve adding at minimum 2/3 to 1 cassette per hour of programming - more if you want multiple copies. So your running costs start out sizable and only grow over time, in search of a highly speculative payoff that at best won't be realized for years to decades.