When you upload a file to AWS, it's using HTTP(S) to post the image via an API. That is a one-time transfer from your server to AWS servers. Once it is there, anytime the file is called, it's delivered directly from AWS via the bucket URL. Meaning AWS is the one delivering the image instead of your server.
If you were to implement FTP, while it would not be difficult to upload the file... downloading it would be much more difficult. Modern browsers no longer allow the FTP protocol in the browser. (For example with Chrome, it was removed in version 59.)
This means IPS would have to each time it was requested, login to the FTP server and download it... then deliver it to the user. That would dramatically increase the work on the server, slow down the delivery of the site (because it has to be downloaded and then sent to the user), and increase bandwidth on both the web and FTP servers.