If I had to guess, it's because the lambda will be expanded at runtime to an anonymous interface implementation. Then either the runtime part is the problem or maybe they use a different, but compatible functional interface for the ad-hoc implementation, which then has no access to default methods on the one you're assigning to.
That is not how lambdas work actually work in the reference JVM.
They make use of invokedynamic, there is no expansion taking place as many think.