Explain Like I'm 5 (ELI5)

You've just invented a really shitty GPU, essentially.

Only some work is easily parallelized (and benefits from it) such as graphics, so people have essentially done what you've described only the cores aren't 8086 but more specialised for graphics math such as matrices and vectors.

As for more standard workloads, we still do a similar thing with server processors that utilize hundreds of weaker cores, but not all work benefits from the kind of parallelism that computer graphics does (or other stuff like AI, blockchain, physics simulations) and some workloads benefit from less parallelism.

But the short answer is that we do already do what you describe there it's just that sometimes it's still faster to have a few complex cores over thousands of simple ones.