Why is "? extends" required for parameter type matching of nested generics?

Question

Question

I understand the reasons that the java compiler will not allow us to treat a collection of type List<String> as a List<Object>. When I write a method designed to operate on a list containing any sort of types, I can get around this by writing

public void doToList(List<? extends Object> list) {
    ...
}

But, suppose that I have nested collections. Since the above method can be called on an argument of type List<String>, I would expect the following method to be callable on an argument of type List<List<String>>.

public void doToListOfLists(List<List<? extends Object>> list) {
    // generates a compile error when invoked
}

However this generates a compile error. In order to make this method work, I needed to write its parameter as type List<? extends List<? extends Object>>. Why?

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Motivation

This seemingly unnecessary requirement has the potential to greatly obfuscate my code. For example, instead of

List<Set<Set<Set<? Object>>>>

I find myself needing to write

List<? extends Set<? extends Set<? extends Set<? Object>>>>