Consider the string needs reversible obfuscation or it won't be usable. The only secure way is encryption but you'd need to properly secure the key (probably using some hardware facility that's physically locked down)
The credentials are stored as a string so you can search the binary for a pattern matching what the credential looks like and it will be in there somewhere.
In client server architecture, the client is always untrusted. An executable shouldn't need to authenticate itself to the server. The executable should authenticate as a user or account using details provided by the person.
In cases like telemetry these endpoints usually accept unauthenticated or lightly authenticated data and perform layers of validation to prevent abuse (and are usually write/append only)
For big flagship services you can usually get pretty good support (EC2, S3, SQS, Lambda)
For smaller/more niche services where AWS stood up a managed version of some OSS it's more hit and miss (like managed RabbitMQ).
In both cases, it definitely helps to have an open line to your TAM and send them case numbers and they'll usually do some internal nudging to keep things moving. In addition, for projects, you can usually reach out ahead of time and get some dedicated SMEs to help set things up/train you.
In either case, hopefully you've never had the displeasure of working with Azure support.
SQS/SNS/S3 are so simple, reliable, and cheap they're pretty much a no brainer. While you can probably run those workloads in Postgres, it isn't designed for those use cases and you'll eventually run into nasty limitations like managing vacuums with high churn tables and slow/complicated backups with big binary blobs.
If you have a good understanding of load up front, however, those are probably non-issues.
I know, I'm mostly being tongue in cheek - the joke is so many companies go straight to complex cloud configurations more for the vibes than the actual practical need; a single box (two for availability) and a solid db will get most sites and businesses very far.
I can run a 64x512GiB server in my home office loaded with NVMe drives for $80/mon (probably cheaper depending on how many years you amortize the server purchase over)!