It isn't. The empty set is a subset - but not a proper subset - of the empty set.
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Because every set is a subset of itself. A proper subset cannot, however, be a proper subset of itself.
The empty set has only one subset: itself. It has no proper subsets.
Since B is a subset of A, all elements of B are in A.If the elements of B are deleted, then B is an empty set, and also it is a subset of A, moreover B is a proper subset of A.
The collection of all sets minus the empty set is not a set (it is too big to be a set) but instead a proper class. See Russell's paradox for why it would be problematic to consider this a set. According to axioms of standard ZFC set theory, not every intuitive "collection" of sets is a set; we must proceed carefully when reasoning about what is a set according to ZFC.
If all elements of set A are also elements of set B, then set A is a subset of set B.