Reading up about type erasure should help you understand this a bit better.
At runtime, the type parameters for a generic class are erased. Meaning, as cletus said, generics in Java are basically syntactic sugar – they are only a compile-time feature.
Since Spring is instantiate objects at run-time, it is actually free to instantiate a Dao of any type – and actually, there is nothing stopping it from creating a Dao and passing in Student types in some methods and Teacher types in another.
So basically the answer is, Spring has no idea that the Dao type is meant to be parameterized and can’t do anything with it.