AFAIK, it takes a minimum of 2 years to produce an Associate Degree RN. Even an LPN takes a year - and most do not have the needed skills for the ICU. So even if nurses had started training in Sept 2020 (because schools still follow a Sept start date) and schools somehow were able to gain enough faculty, room, etc to dramatically increase their class sizes, they still wouldn't be ready until next year. And they would be "green" nurses with little experience.
And yes, there may be current nurses who are willing to travel and earn extra money, but there are more who have families, etc that they do not want to leave. Especially now that they know this could be going on for months.