Because that object is of a type where that member may or may not exist. That is literally the exact same behaviour as Java or C#.

If I cast or type check it to make sure it's of type Bar rather than checking for the member explicitly it still works:

And when I cast it to Foo it throws a compile time error, not a runtime error:

I think your issues may just like in the semantics of how Type checking works in JavaScript / Typescript.