This always gets brought up as a use case, but how many people are building non-trivial codebases where compile times matter? If build times are under 5s, near perfect parallelization only nets you a 1.25s gain between 3700x and 3600.
You want to see a preview as quickly as possible. Best if you can keep a live view on a second monitor.
The only scenario I can foresee where people wouldn't be doing parallel builds is if they are just working on their personal side project with no other people involved, where the need for parallel builds is non-existent either way. You don't want to have one giant monolith project for your whole complex web app, it becomes a nightmare very quickly.
I dread having to update the compiler or some core "used nearly everywhere" header file, and wait a whole another 28 hours...