You’ll probably discover only 1 or 2 pairs were ever used too. Telcos were notorious for putting in ludicrously complicated wiring for what was in reality a simple two wire analog circuit or two.
Future proofed for stuff that never happened or was just about making it look more complicated to justify the line rental fees.
Well, there was an element of that involved, but they also tended to just throw tons of spare pairs in with an idea that some future tech might use them. At the time PCM based circuit switched data was being developed and envisioned to launch as some point. It would ultimately launch as ISDN in the 1980s.
Local ISDN S-Bus wiring usually ran with two pairs and an additional two pairs for power. It was rarely deployed in homes, other than in a few European countries, but it (or proprietary versions of it) were commonly used for office telephone systems.
Rapid advances in multiplexing in the 90s saw DSL emerge as the most common way of getting fast data into homes and small offices. That only required a single pair and then telcos shifted to fibre to home. So it’s all redundant.
You’ll see plenty of home phone installations that used CAT3 or even CAT5 in the later days of PSTN with tons of spare pairs.