In normal times, true. However unemployment insurance has been significantly boosted by the CARES act during the pandemic
There are a few exceptions under the CARES act if you can't work because of something directly related to COVID, e.g., if you're a parent who has to take care of children because schools are closed. Being afraid you might catch the coronavirus at work is not a valid reason and would not make you eligible for unemployment payments.
Even among parents, I doubt that all of them would leap at the "opportunity" to quit their job just so they can take home slightly more money for a period of four months (which is all the CARES act covers). Surely most people must realize jobs are going to be extremely scarce four months from now, so if you have a job right now you probably should think very hard before throwing it away.
You're right, they can't.
However, the 26 million Americans who have been laid off from other jobs have negative incentive to sign up for a new job with Amazon because the benefits are tied to staying unemployed. That exacerbates the problem for Amazon.