AWS Resource Explorer does not see all Resources

You're correct in observing that AWS Resource Explorer doesn't include all resources in its search results, even when you create a view that includes all resources from a region. This is a known limitation of the service.

While AWS Resource Explorer aims to discover and index most resources in your AWS account, certain resource types are not indexed. Unfortunately, HTTP API Gateways are among the resources that are not currently indexed by Resource Explorer.

The reasons for some resources not appearing in search results can vary. In some cases, it might be due to privacy concerns or the nature of the resource. For example, Amazon S3 objects and DynamoDB table items are intentionally not indexed by Resource Explorer.

As for a comprehensive list of resources that can't be included in a view with AWS Resource Explorer, AWS doesn't provide an exhaustive list of excluded resource types. The set of supported resource types can change over time as AWS continues to expand the capabilities of Resource Explorer.

If you're looking for specific resources and they're not appearing in Resource Explorer, it's best to consult the AWS documentation for that particular service or reach out to AWS Support for the most up-to-date information on Resource Explorer's coverage.

It's worth noting that the limitations in Resource Explorer can indeed affect other services that rely on it, such as AWS Resource Groups. If a resource type isn't indexed by Resource Explorer, it won't be available for use in Resource Groups either.

For critical resources that aren't showing up in Resource Explorer, you may need to use alternative methods for tracking and managing these resources, such as service-specific APIs or the AWS CLI.
Sources
Troubleshooting Resource Explorer search issues - AWS Resource Explorer
Terms and concepts for Resource Explorer - AWS Resource Explorer