Sorry to be stubborn, but I don't understand why Amazon is available on my other devices here. Amazon does not seem to have a policy blocking Prime Video in Israel.
You'd have to ask Amazon. I can't speak to why Amazon would choose to not support Roku there or in other places where they choose to not support.
I don't know what other devices you're using, but if they are devices that are sold and supported by their manufacturer in country, then that could be it. Roku doesn't sell devices everywhere.
Here is where Roku is sold and supported:
If you aren't there, you won't get support from Roku. Well, they actually have attempted to help people outside those regions on this Forum, but the bottom line is, Roku is officially unsupported there.
I suspect, but don't know for certain, that Amazon won't support their service on devices that aren't supported by the manufacturer. You can test that, though. I can't, because I'm not there. But, you could attempt to find another device that is not sold in country and try to use Amazon on it, without attempting to get around Geo-restrictions. Then you'll know.
Regardless, whether you like it, whether you understand why, none of that matters to facts. Facts are stubborn that way. Amazon and all app developers make the call about whether or not their services will work on Roku in any particular country. Roku doesn't make that call. If Amazon updated their Roku app to allow it, it would work.
Keep in mind that there are devices in the United States that Amazon won't work on. They're older devices, to be sure, but app developers determine what devices they'll support, and where.