I think it's two main reasons: money and specialization.
1) To obtain a PhD you need to go to school for another 4-7 years, during which you're paid very little. After you graduate, there are still few job options. Often you will have to move to find a job. If that job is in academia, you will continue to paid very little. Working at a biotech company is financially ideal, but those jobs are hard to find.
2) Imagine you spend 5 years getting a PhD in biomedical engineering, and your speciaty is imaging of the aortic valve. You know everything there is to know about it, and you've written papers about it. But unless someone is hiring aortic valve imaging specialists... you'll have a hard time finding a job.
It's overall not a great return on investment of your own time. For some people academia is a terrific life and they're willing to accept less pay, but for other people its not worth it.