I believe the reason for the recommendation of DigitalOcean in the standard install is the relative simplicity. The goal is to make the process as straightforward as possible so that people can get on with using the product, rather than get mired in additional steps such as network ACLs, object storage, and caching.
As Richard has pointed out, other guides do exist, including the one you’ve requested, but these are more complicated and error-prone. I’ve amended your title, Discourse doesn’t deter against installation on Amazon, it’s just not optimal for first-time installers.