The president has stated he is not ready to ask them
"He's casting doubt over whether he will actually implement a move to invoke the Defense Production Act -- that he signed on Wednesday and that gives him authority to order industry to work towards homeland defense and national goals. In this case, it could speed the production of badly needed ventilators, masks and other supplies for hospital workers" - https://www.cnn.com/2020/03/19/politics/donald-trump-leaders...
FEMA and the Army Corp of Engineers have said the same thing. For some reason they are not being mobilized while CA, WA, NY/CT/NJ are dealing with a crisis and have asked for that help, formally and officially. Meanwhile, ICE is fully deployed and arresting people in hospitals, then concentrating them together without adequate separation.
GM and Tesla (and Ford, and all other domestic manufacturers that have done so) should be saluted for volunteering to help. But there is a slow-walking of the response and something very, very rotten is going on in some areas at a coordination level in the US.