Not true at all! Most of the HTTP APIs, and a good chunk of the webapps, that I've worked on can be defined as a combination of an API spec that carves out valid and invalid behaviors, and a set of behavioral tests for the workflows that the client users care about. Working from a codebase which is generated from a spec document (e.g. OpenAPI or gRPC) and use of tools like https://pkg.go.dev/net/http/httptest and https://bun.com/docs/test/dom makes this a pretty achievable goal in practice.