Same! I bought a chrome cast without realizing I couldn’t watch Prime on it, then I learned about the Amazon/Google dispute. I felt so disappointed in these companies.
Amazon has added casting support to the prime video app.
Why did they withhold support in the first place? I had assumed it was because they wanted to sell fire sticks. It's not in Google's interest to make Chromecast less capable.
This was honestly never a big deal for me because I could always cast from my browser on a laptop. But, yeah, I've observed that some of the Apple/Google/Amazon incompatibilities seem to have gone away over time.
I believe Google didn't permit FireTV to carry YoutubeTV because Amazon wouldn't sell Google Chromecast in their store (because it competed w/FireTV I guess).
Why did they withhold support in the first place? I had assumed it was because they wanted to sell fire sticks. It's not in Google's interest to make Chromecast less capable.