It's not a transitive dependency, it's just literally bundled into nextjs, I'm guessing to avoid issues with fragile builds.
why is it not normal for CI pipelines to cache these things? its a huge waste of compute and network.
It's certainly not uncommon to cache deps in CI. But at least at some point CircleCI was so slow at saving+restoring cache that it was actually faster to just download all the deps. Generally speaking for small/medium projects installing all deps is very fast and bandwidth is basically free, so it's natural many projects don't cache any of it.
These often do get cached at CDNs inside of the consuming data centers. Even the ISP will cache these kind of things too.
Beyond that, I think there is good reason to believe that the number is inflated due to automated downloads from things like CI pipelines, where hundreds or thousands of downloads might only represent a single instance in the wild.