This is the end-goal. The delivery of synthetic genetic payloads in a live organism is still a bit tricky. Certain therapies do do this by injecting a synthet

This is the end-goal. The delivery of synthetic genetic payloads in a live organism is still a bit tricky. Certain therapies do do this by injecting a synthetic 'virus' directly into your bloodstream. However, since it's so easy to extract T-cells, the risk of withdrawing blood, engineering it, & replacing it, it is worth the extra time to guarantee the delivery of the genetic payload is correct and thorough. As the sophistication of genetic delivery improves, direct delivery will absolutely become more common - and in fact will likely be a required technological step to curing harder-to-reach cancer and genetic abnormalities. That's why the first gene therapies are for the immunologically privileged, and easily accessible sites of the blood and the eye.