I think this comes down to how many in the US see jobs in the general. A job working in an Amazon fulfillment center is seen by most as a kind of "stepping stone" job. Literally anyone can get the job very easily, and as a result its not considered to be a "good job" in the first place. There is an understanding that you work this type of job if you are fresh out of high school with no experience, or you don't have any skills so you can't find better work, etc. So it's considered okay to suffer in the short term, as you don't plan to stay in this type of job for the long term anyway. In fact Amazon leans into this by offering programs to pay tuition for fullfillment center workers to learn tech skills, etc. Part of the draw of the job is that you can more easily pivot from the role into new better roles.

So with this background its not hard to see why workers don't bother to join a union or invest that much in improving working conditions in their current role. They want to be moving on to another job or better role in a year or less anyway.

Disclaimer: I am an Amazon employee, and I am speaking from my own personal viewpoint, not for Amazon.