I'm in India and we use primevideo.com. It does come as part of the Amazon Prime subscription.

That may be because Amazon Prime doesn't make sense in some countries. Example: Free two day delivery doesn't make sense if you can expect free/cheap next-day delivery from local webshops.

Other Amazon Prime benefits requires that Amazon have a local presence in your country or area, so it makes sense that it's limited to countries where Amazon at least have distribution centers.

Just to add to your comment, Amazon Prime also doesn't make sense in countries with a fairly poor Amazon presence / high shipping costs. E.g. Canada.

I'm in Canada (lower mainland) and have Prime. The free same/next day shipping makes it worth it for us. We don't order a ton, but probably 15-20 times per year is enough to justify it.

It also makes sense because primevideo.com looks like it was built from scratch and isn’t integrated in the “normal” Amazon.tld that’s not available in a lot of countries. It was very buggy in the beginning and incrementally got improved, they probably couldn’t move that fast if it was integrated into the main site too.