May have multiple reasons:

1) Some people just like the infra work, so they presume everyone likes as well.

AWS eliminates/simplifies a good portion of it.

2) Some feel a sense of superiority saying they can pull off an entire infra on Hertzner.

They know many younger engineers can't, since they started with AWS in mind.

3) AWS has a terrible reputation as an employer.

Perhaps some these anti-AWS blog writers might have worked there, know someone who did, or heard one of the AWS PIP horror stories...