The problem is that many algorithms simply cannot be parallelised well and that ever since the 90s the iron is cheap whereas the programmer's time is expensive.

The problem is that many algorithms simply cannot be parallelised well and that ever since the 90s the iron is cheap whereas the programmer's time is expensive. So instead of spending weeks or even months on trying to squeeze the last bit of performance out of an application that's "good enough" performance-wise, developers can use that time to roll out features or fix bugs instead, i.e. generating value for their customers. It's simply a question of economics.