In Java, all method dispatch is virtual (dynamic), but in C#, methods being virtual is an opt-in, so intercepting and mocking such calls requires a lot more effort.
It's still possible, mind you. TypeMock has been offering this exact ability for C# for many years now. But the free TDD frameworks generally didn't have this.
Java's method dispatch is more complicated than that due to JIT compilation. The affordances are those of dynamic dispatch, but hot Java method calls will not go through a vtable-like lookup equivalent unless the code actually sees a need to.
Indeed. Both OpenJDK HotSpot and .NET RyuJIT perform guarded devirtualization of monomorphic or polymorphic with few instances callsites. OpenJDK also computes optimized call table for megamorphic callsites, which .NET does not need to do for virtual calls, it does have something similar for un-devirtualized interface calls however (virtual stub dispatch[0]).
This is not necessarily zero-cost however - if the compiler cannot prove specific type members being invoked, it has to construct an execution profile and then apply it to subsequent compilations, and also emit a guard when doing dispatch on those.
It's still possible, mind you. TypeMock has been offering this exact ability for C# for many years now. But the free TDD frameworks generally didn't have this.