It might be better for these two to merge than for either (or both!) to be subsumed into those much larger firms. This is why FTC allowed Sirius and XM to merge: to keep them both out of the clutches of larger firms who would have seriously considered killing satellite radio altogether.
[EDIT:] Of course, this merger may just be a ploy to drive up the price of a future merger with one of the larger firms...
Cognito and Firebase are bush league by comparison. They can do the basics well enough if you have the right integration engineers. Okta and Auth0 are light years ahead.
The difference is that Okta/Auth0 is never going to be the only piece of a solution. With AWS it's more than just Cognito, you have to consider IAM and SSO as part of the equation as well. And if you're a pure AWS shop the AWSness of Cognito (or its direct support in API Gateway, etc.) might make you prefer it to Okta or Auth0 regardless of feature parity. For Google the key asset is really Gmail/GSuite/Workspace, which is the primary identity provider for many, many organizations (and the sole identity provider for most of those). However kludgy Google's built in SAML stuff is there is a huge value in only needing to deal with one entity.