In the 90s, even right at the end of them but especially earlier on, space was
far more expensive – even a miniscule saving could be worth a bit of faf (faf in terms of dev & admin time, and a small efficiency hit at the CPU). Also, RAM was expensive and shared libraries allow some of that to be saved too if your OS is bright enough. Back then the benefits really were apparent, especially for common core libraries (libc in particular).
Of course these days storage and RAM are both massively cheaper, and there are a plethora of variants even for what might previously have been the core version of something, so the benefits are minimised.
We've seen similar with JS libs over more recent years: first everyone had their own copy, costing bandwidth and meaning every site/app dev had to update their version when there was a security issue. Then CDNs became a thing for libraries & such, reducing the bandwidth cost and (unless you pinned at a specific minor version) making security/efficiency updates easier. Now as shared sources for JS and the like that are potentially a privacy or security issue, and treeshaking & such can reduce the payload for most uses, we've swung back and (almost) everyone has their own version, as-is or crunched into a bundle.
Package management, the likes of which wasn't available in the 90s, deals with the updating of JS libs even if everyone has local versions (they get updated on next build, unless holding back to full test a new release), but that in itself is a security concern due to supply-chain attacks.