Comment by sublinear

> In such environments programmers are offered a mechanism that has many downsides of parallel / concurrent programming (eg. unpredictable order of execution) without the benefits of parallel / concurrent programming (ie. nothing actually happens at the same time, or only sleep is possible at the same time etc.)

From the developer's perspective it's a massive upside to not have to manage low-level details and just define how the event loop will call their code.

Any modern web browser has plenty of parallel execution behind the scenes, but the developer (and user) will just see concurrency which is much simpler to reason about. The order of execution doesn't matter if the things being executed aren't dependent. What matters more is that there's only one thread to think about. If they are dependent they shouldn't have been parallelized in the first place, so they're not.