Because they can't. Not easily, not certainly.
If you've been given direction to quickly deploy a warehouse, being able to sign a single agreement and know you have your work force is valuable. (The alternative involves creating a hiring pipeline, staffing it, running it and lining all of that up with everything else. If you're a scaling company that's a lot of non-core B.S.)
Put another way, all business is bundling and unbundling. A recently-fired union is a bundle of labor. That isn't valuable to everyone. But it's definitely valuable to at least some labor buyers.
Alabama lost a Toyota plant to Ontario because literacy is so poor they would need to translate their documentation into Ikea-like pictograms.
https://www.motortrend.com/news/suv-shuffle-mdx-production-m...
The Toyota RAV-4 decision was made back in 2005. In don’t think education has improved in Alabama since. Also, IIRC, the higher Canadian wages were partially offset by not having to provide health benefits.
I cite it as I’m somewhat familiar with it as a friend and colleague worked for a supplier pricing out a in-region facility.
Given the current labor market, that union could probably bulk sell its labor to someone else willing to come in and pick up where Amazon left off. This is another, less-mentioned benefit of collective bargaining: it aggregates labor.