Because it’s not.
Indexing is covered by IList. IEnumerable means “I have some of the powers of IList, but not all of them.”
Some collections (like a linked list), cannot be indexed in a practical way. But they can be accessed item-by-item. IEnumerable is intended for collections like that. Note that a collection can implement both IList & IEnumerable (and many others). You generally only find IEnumerable as a function parameter, meaning the function can accept any kind of collection, because all it needs is the simplest access mode.