Usually, it’s because someone bought into GAE, and needed some flexibility for extensions to that app.

Cynically, fairly typical vendor lock-in.

afaik there is no alternative to BigQuery on AWS? All alternatives are less managed and require more administration. But I never looked too closely.

Snowflake. It does not work internally the same way as BigQuery but is substantially equivalent in user-visible functionality.

kubernetes is nicely integrated in GCP

OCI as in Oracle Cloud Infrastructure?

Yep, it's the deep-pocketed enterprise's Hetzner or DO.

The sales slogan is something like, "All the costs of AWS, all the automation of a bare metal box!"

Ironically, more suitable to manually point-and-click or phone-call managed "lift and shift" than the infra-as-code clouds.

Oci is much, much cheaper than aws. Data transfer for example is 10x cheaper.

Azure doesn't exist, apparently.

Azure I'm not too knowledgeable in terms of polish, but pricing wise it's similar to aws and gcp.

not if you're trying to use compute in Germany:

https://news.ycombinator.com/item?id=33743567