Google Cloud Load Balancing docs explains the default HealthCheck interval is 5 seconds.
https://cloud.google.com/load-balancing/docs/health-checks#create_a_health_check
also I found that NEG's default interval is 15 seconds.
Why is reason? Thanks.
The difference is due to the different native environments. Google Cloud Platform has a default health check interval of 5s. While Kubernetes has a default pod liveness check of 15s. The NEG default which you have observed is also 15s to match the pod liveness check, both coming from Kubernetes in this case.