They invested in and are partnering with Here for maps and navigation. Having worked there, providing global maps and navigation is a very non trivial thing to pull off; which is why there are only handful of companies doing their own maps. It took Apple and Google many years to stop depending on third party maps that they were licensing from Teleatlas (now owned by Tom Tom) and Navteq (now part of Here), which were at the time the only suppliers of maps and navigation world wide. Apple is still in the process of shifting away from Tom Tom I think. Ironically both got more serious about doing their own maps after Nokia bought Navteq.
So, Amazon did the smart thing and simply partnered instead of reinventing that wheel so they could have a maps solution for their Android fork and not depend on Google Maps while still implementing the relevant APIs for app compatibility. Here is in fact using AWS so that ticks that box ...
OSM is nice but entrenched in licensing issues that are preventing adoption by big corporations. That's the reason that you'll be hard pressed to find anything but niche products for navigation based on OSM. It's a great solution if you are not mixing your own data sources with what OSM provides but that is not an option for most commercial players and OSM licensing is all about preventing exactly that. Microsoft had OSM maps for a while on Bing but they couldn't do any fancy features based on their own data on top of it so it got scrapped.