Why should I get diagnostic testing if it is so risky?

Benefit vs. Risk. For any test or treatment, the benefit that would result must be weighed against the risk of the test or treatment. If the information obtained from the test will make a difference in your health and the risks are low, then you would do the test. If the information will not make any difference to your health or won't be used, then the risk is not worth it.
Making decisions. Diagnostic testing has risks. It also has benefits: it helps us know if there are specific problems with your pregnancy. The real benefit is what you do with that information. If you were having a baby that had challenges, would you do something different? Would you change your job or where you live? Would you terminate that pregnancy. If not, maybe the risks are more than the benefits.