Why do communications between internal services need authorization like oauth if the outside world can't access the apis directly?

Why do 2 or more internal services still need token auth like oauth2 to communicate with each other if the outside world doesn't have access to them?

You don't need OAuth2 or token authentication, but you should use it. It depends on how much you trust your traffic. Now in the "cloud age" it is common to not own your own datacenters, so there is another part that own your server and network hardware. That part may do a misconfiguration, e.g. traffic from another customer is routed to your server. Or maybe you setup your own infrastructure and do a misconfiguration so that traffic from your test environment is unintendently routed to your production service. There is new practices to handle this new landscape and it is described in Google BeyondCorp and Zero Trust Networks.

Essentially, you should not trust the network traffic. Use authentication (e.g. OAuth2, OpenID Connect, JWT) on all requests, and encrypt all traffic with TLS or mTLS.

Couldn't their apis just filter internal IP addresses instead? What are the risks with that approach?

See above, maybe you should not trust the internal traffic either.

In addition, it is now common that your end-users is authenticated using OpenID Connect (OAuth2 based authentication) - JWT-tokens sent in the Authorization: Bearer header. Most of your system will operate in a user context when handling the request, that is located within the JWT-token, and it is easy to pass that token in requests to all services that are involved in the operation requested by the user.

For internal services it's usually less about verifying the token (which in theory was already done by the external-facing gateway/api), and more about passing through the identifying information on the user. It's very common for even internal services to want to know who the requesting/acting user is for permissions and access control, and sometimes it's just easier to tell every service creator who needs user-scoping to accept a JWT in the Authorization header than it is to say, "look for the user ID in the X-COMPANY-USER-ID header".

You can implement very granular role based access control(RBAC) on the apis exposed by Microservices using Oauth which you can not do using filtering IP address.