Cloud providers (particularly the hyperscalers) are ultimately bundles of multiple services. Given that the hyperscalers do almost everything, you could extend this point in a variety of ways: why do companies already on AWS bother to use MongoDB, Snowflake, or even GitHub when DynamoDB, Redshift, and CodeCommit exist?
The answer tends to boil down to a combination of developer experience, performance, and pricing. Fwiw the actual platform offerings on GCP are also more intuitive than the equivalent services on AWS + Azure where most businesses/startups are hosting services
Edit: cloud vendor lock-in is also a very real phenomenon regardless of how much it just "looks like" all cloud providers should be easily interchangeable. Needless to say, the incentives when you make money selling compute are to keep people on your stuff
The answer tends to boil down to a combination of developer experience, performance, and pricing. Fwiw the actual platform offerings on GCP are also more intuitive than the equivalent services on AWS + Azure where most businesses/startups are hosting services
Edit: cloud vendor lock-in is also a very real phenomenon regardless of how much it just "looks like" all cloud providers should be easily interchangeable. Needless to say, the incentives when you make money selling compute are to keep people on your stuff