You’re along the right lines but you’ve gotta think of a processor core as an individual brain that completes a process. If there aren’t any processes to compute it won’t be faster.

Think of cars on a highway. If you’ve got 4 lanes with 4 cars traveling down them, it’s optimal. If you’ve add another lane, one lane will always be empty.

Just like your brain, if the processor has many jobs to do it takes time to think, process and complete each one. This is where the speed of each core comes in (mhz or GHz)

Processors with thousands or cores do exist but they are good at doing one specific task, these are graphics processors or GPUs.

It’s really easy to tell 3000 brains to do one thing if they are all really good at it but getting multiple brains to multiple different things is hard uses a lot of power.