If you're running on AWS it's really nice to have everything preconfigured for things like logging to Cloudwatch, and not having to mess around with kernel optimisations.
Ubuntu considers AWS/GCP to be certified platforms so the server is fully tested on AWS as a hardware platform and there are specific kernel/toolchain/user-space optimisations done to make it run well [0].
The advantage of one of the existing distributions over Amazon Linux is that they are available and tested across multiple clouds, hardware platforms and range of software.
However, they do have images with other distros also well preconfigured though don't they?