OpenAPI/Swagger 3.0 Support

They should. They have code but have intentionally left it out from Open source edition.

In 5.7.0 release notes:

Built-in OpenAPI (Swagger) support

Now, to work with OpenAPI/Swagger definitions, you do not need to use plugins. SoapUI supports it out-of-the-box. Additionally, we've improved integrations with SwaggerHub to make it even more convenient.

They should have included OpenAPI 3.x support in that change. I guess it could happen eventually when some other vendor Open source tools gain more weight. OpenAPI is kinda industry standard so it should be supported in regular basis and not seen as some premium feature.

In the meanwhile I would suggest that community creates a plugin for OpenAPI import functionality.