Projects with 100+ tracks are not uncommon. Sampler/rompler of a single virtual instrument can play 10+ sounds simultaneously. Playback of an orchestral score with virtual instruments can easily go over 250 simultaneous sounds, so just a real-time playback (without any additional processing) would already be a challenge.

In those contexts samples are loaded once and then kept in RAM.

No, actually only first few dozen KB of each sample are usually preloaded into RAM. The rest is streamed from a SSD. One library of an orchestral section can have 100+ GB of samples. You wouldn't fit all sections in 128GB of RAM.

I'm quite aware of the size of large libraries, I own a number of them.

Part of why I built my current machine with 64GB of ram