People do leave Amazon warehouses. At an astounding rate, far beyond other similar employers. Also, their injury rates exceed other similar employers as well. (https://www.nytimes.com/2021/07/04/insider/amazon-workers-investigation.html paywalled)
Yep, Bernie Sanders through retirement savings, working as a Senator and with his wife, have a total amount of assets around 2.5 to 3 million (including his house). Which is 9 million below the average Senator. (https://ballotpedia.org/Net_worth_of_United_States_Senators_and_Representatives). He's not stupid about money, but he is very, very consistent in his support for labor rights.
I am glad your mother in law finds it a good place to work. But, this anecdote is merely that. If Amazon was truly a great place to work, they would welcome the chance to show that to Congress and the call for unionization wouldn't exist.
I use Amazon a lot. I just want it to do what all companies should, which is invest more in workers and the communities they serve versus maintaining short-term market performance and untenable wealth accumulation for a very few while denying that growth and wealth to every generation since then. Despite them being more productive than ever.
Finally, if you don't like working in a union job in the public section, why not get a new job? Like those that don't like working in an Amazon warehouse.